I think the whole sense of Astartes is not to make them ACTUALLY INVINCIBLE. I think their use is more of being an elite against the strongest enemies there are and therefore be more an INSPIRATION to mankind than being actually the ass-kickers.

Sure they kick butt, nowhere on the levels of their daddies (Primarchs) but they still kick good (heretic & xenos) butt. In the end though they cannot fix and protect everything.

I may recall Tarsis Ultra and the Hive Fleet Leviathan at this point. 2 Companies of Space Marines were sent to protect the planet among with 2 IG regiments (local regiment & deathkorps of krieg). So in basic roughly 200 Space Marines + Hundreds of thousands of regular human warriors. So what the Ultramarines did during the training of those men for example was to say all the time how crappy they are, that they need to do better or they die, etc. And then..... Right BEFORE the attack of the Tyranids started the Ultramarine Seargent who trained them told them how great they are and stuff like that and BAM they were motivated to nowhere and fought harder than they ever might have in any other battle.

So I think that the Astartes themselves are BELIEVED to be as good as invincible and with unshakable faith to slay their enemies with their super-strength and other abilities that comes through the bio-engineering, so that they would fight while being convinced that they would go victorious out of fights with no matter how the odds are. This way they can fight and fight and fight and fight without ever giving up (mentally) like PDF love to do and receive a headshot of their commissar in reply.

So the Astartes really are a tool of inspiration to all humans of the Imperium + taking care of the REAL Threats, which outcomes are critical. Armageddon is a good example where even the super-duper-mega-elite of the Imperium can fail. And some of the IG books a good example where the Imperial Guard actually can succeed without additional help (I do not know so many IG battles where no Astartes participated so I have no idea yeah..... ^^)

People like SMs for multiple reasons, they are Heroic and powerful etc, so that gets a lot of people, but they are also simultaneous a little sad.

They have literally given up everything they were or could have been as a human, live a loving life, have children, be at peace at least dome of the time, for lives of eternal war an bloodshed, dying brutally with only their few brothers to mourn their passing.

Most do not seek power, or glory, or conquest for themselves, they serve the Imperium (as corrupt as it is on the inside) and humanity as a whole, knowing that without them mankind would have been consumed by the darkness.

Yes they can be corrupted, but so can any man (apart from most grey knights). They were never meant to be incorruptible, and they still give in to human failings. But that is why we love them. They have the power, and the greatest reasons in the world to despise the common man for his multitude weakness, but they (on the whole) do not, they rise above these emotions and fight on.

They may fail, and some fall, but they sell their lives with an ocean of their enemies blood, and that is all we can ask of warriors like Space Marines. (And if you think what has happened to a SM is gross, have you ever seen a tech priest, that stuff makes the Borg look good).

That is heresy to say. Space Marines are still human. That is exactly what the Codex Astartes is supposed to teach you besides battle tactics and such. You are as human as the IG, as the pilots and Navy which take care of the Space battles. Also that is why Astartes have emotions, if they would be no longer human then they would be without emotions. And therefore they would be nothing but Killer Machines and that is exactly NOT their purpose.... Read the Ultramarines novel "Courage and Honour" it has a good part in there where Uriel realizes once again what the Codex Astartes is about.

Doesn't look like the book "Courage and Honour" to me The blood ravens are no regular astartes they are an ABNORMALITY. .... Read the first Blood Ravens book from C.S. Goto at the end where angelos even recruited a damn mutated Marine who had tentacles instead of a regular hand.... Yeah great going taking Blood Ravens as the standard ^^

Out of universe, the answer is simple. Just like any other race something draws you to a particular faction. Granted SM have a lot more fluff as the poster boys/focus of the universe in novels predominantly.

Otherwise...why any of the races? If you get right down to it, Eldar, Tau, or the Imperium are the only real choices. Chaos, Ork, Necron, and Tyranid, have no real future should they become the dominant/sole force in the universe for it's individual.

Chaos:
You might obtain power as Chaos, or you might even have a measure of freedom...but it's as much an illusion as the Imperium can be at times. You're never free as Chaos, someone is pulling strings you can't see, and they all end up somewhere being pulled down the line by one of the Chaos gods. Chaos taking over the universe would lead to basically hell for everyone not a Daemon Prince.

Necron:
The Necron would destroy anything not of themselves as well, their lowest members have no free will and the higher ups play politics between themselves forever.

Orks:
Orks...are Orks. I guess technically being an Ork means life is probably awesome. I mean, fighting everywhere and lootz for EVERYONE! Except, you're genderless and most never make it past being canon fodder. (Probably still awesome for them.) They'd give the universe nothing however, and it would eventually be taken over by Chaos after massive infighting occurred once they started becoming incredible dominant.

Tyranids:
Tyranids just want to eat everything. Life really can't suck when you don't have a life of your own, but then there's nothing to live for.

Now as for the other groups, Eldar, Tau, and Imperium, any one would be preferable to the alternatives. They all have things in common but that's mostly cause they can't ever get along due to GRIMDARKNESS for all.

Eldar:
Barring the "We're superior to you and the master race" (Which I admit isn't something that is unique to Eldar, they just often play on it quite well...and not all do,) they're an interesting race that once had a mighty star empire. It'd be great fun to be a long lived and most likely very psychic individual...iiiifff Chaos wasn't poking at the edge of your thoughts waiting to troll you harder then the internet. Very advanced and able to field insane technology, unfortunately don't play well with others and kind of birthed a Chaos god/Eye of Terror. Too few of them to really accomplish anything in the ways of bringing themselves back from obscurity especially with their gods being devoured. They fight on bravely still standing against the evil they unleashed, risking their very souls in addition to their minds and their lives.

Tau:
The Tau would be a much stronger pick then any faction if they weren't just like the Eldar and Imperium. Our way or the highway and if you don't like it I've got a really fancy gun to change your mind. Don't appear in the warp so Chaos is boned if the Tau ever become the most dominant force in the galaxy. Only way for that to happen would be for the Eldar and the Humans to die out after the Tau conquered them, because neither would last long as they'd both be relegated to being second class citizens then fade away. Still, very advanced for a young race and only their limited numbers really hold them back from being unstoppable.

Space Marines:
Saved this for last. Why a Space Marine? Yes, the Imperium is xenophobic and wish to dominate and destroy anyone that is not Human. The Imperium is stagnant in it's technological advancement and it had this little thing called the Horus Heresy. What the Eldar started by birthing Slaneesh the Imperium's forces bolstered Chaos by falling prey to it's machinations. Even with half it's (surviving) Legions falling to Chaos they still hold the line. No other force in the entire Galaxy could have stopped the Tyranids, Orkz, Necron (which...we kind of woke up since we needed more things to kill <.<) and Chaos forces for as long as they have.

The Eldar are a doomed race, the Tau's Greater Good is an attempt to spin conquest in a positive light, and the Imperium has little freedoms for the cost of "security."

As for Space Marines having no choice....you forget that for some, eternal servitude is their choice. Their reasons can vary, some are so indoctrinated that that is the only "choice" while others do take pride in defending their Emperor and his people. Why can't a Space Marine be proud to spend his life in defense of those that cannot do it themselves? He might no longer be strictly human, but some recognize they've become living weapons and tools to keep the Imperium alive so that there might finally be a brighter day. It also doesn't mean they don't have a full spread of human emotions and that's what allows them to stand fast against the hordes of enemies in the galaxy, but they still have a duty that cannot be shirked if the Imperium and humanity is going to survive.

So honestly it comes down to three. Any one is just as good as the other because they're all flawed in specific ways and all have good aspects.

I choose Space Marine, namely the Space Wolves, because I love the look and lore of the Wolves. I love the fact they have enemies on all sides and are just as flawed as all the other races and yet they still hold and can perform noble deeds. Might they get a lot of story help due to being the forefront race? Sure, but we're also humans and it's kind of natural in most sci-fi (not all.)

Astartes are not human... We are more than man... But we are to defend humanity. We are no longer human because we are genetically altered to the extent that we are not fit to be human... But that does not mean we do not retain our own humanity... Sure we are Astartes... But we have retained all our mentality from when we were humans... What you speak is both truth... And misinformation... Many Astartes lose their humanity... And have become detached from their roots... And that... Is when heresy takes root. To be better warriors... We must understand our role, our history, our biology, our faith, and especially our duty... Heed the words of those who came before you...

Astartes ARE humans I won't move from that statement.

Of course much is different but they are still in the same species. Bio-Engineered to be improved is not actually being "another race" the Adeptus Astartes are as human as the Adeptus Sororitas are.

Also I have NO source found that in some way states that Astartes are unhuman. Rather that they are the guardians of mankind and the last stand between the Imperium of Man vs. Renegades & Xenos. They are there to not lead the destiny and fate of mankind, but to protect the Imperium so it may unfold itself.

Brother... We are not human... We have undergone extensive modifications... Far more than any other Members of the human race... We have far too many organs... We are physically different... And I have not read any source that says otherwise... There is a reason we are referred to as "post human" meaning that we were once human, but not any more...

1. Stop that ... crap please on having a proper discussion it's just so damn weird lol

2. "Brother"? We are having a discussion no RP lol!

3. Having more organs doesn't make you less human automatically. Physically different? Yeah more muscles, more endurance, etc. In basic just an upgrade of what the human body can do already just stronger.

4. Astartes have still to bear the "weaknesses" of humans:
- Sleep (though only minimal it's still there)
- Bleeding (an Astartes might not die from this, yet it's still there which gives the feeling of being mortal)
- Emotional (as an Astartes you still feel compassion, pain, fear, etc.)
- Limited thinking (every Astartes can only think as far as possible and therefore is limited in capacity of thoughts, tactics, etc. Just like most other beings)
- Own Decisions (every Astartes still can make own decisions which can range from not following the codex to become a traitor or renegade)

So my list of above (except for limited thinking perhaps because I do not know really how to put it in proper words) just shows how human an Astartes can be. The thing is just that an Astartes is around 100-10000 times more effective than a usual human. E.G. physical strength and endurance. But they are not "Post-Human" some may describe it like that but that is rather a feeling than an actual biological statement.

The only post-human creatures are those machine-like guys which name I forgot (locari or something like that), Servitors (not sure) and Dreadnoughts (some of them are really not human anymore or completely mentally intact).

To be practically "no longer human" would need to be a whole other species, but they are not they are just "Homo Sapiens 2.0" with additional organs. But the basic of their physiology and mentality is still the same. (Rage, Adrenaline, Fear, Vengeance, being jealous, humor, etc.)