Never forget - 40k was never supposed to be sci-fi, its just too far from all possibilities. 40k is science-fantasy-dystopian future.
To me the biggest question is, can humanity "win" in the Warhammer 40k universe, lore wise? Everything points up to the total colapse and the end of civilization (both humans and eldar actually). Did they ever get close to saying what the conclusion would be?
If in the future i can play with an inquisitor or a vindicare assassin...... good bye LSM i have my own heroes in other places
Overall yes, but there are heroes among humanity from what I read this far. People that are genuine good and all that.
It is the Guard, hands down. Why? Because Space Marines aren't brave, they are fearless. Bravery and Heroism are about conquering your fears to face the enemy. A Space Marine has been genetically engineered to the point that fear as a man would feel it isn't even a factor. They're psycho-indoctrinated super-soldiers crafted into biological war machines. Are they dangerous? Of course. Do they face threats few could even dream to defeat? Definitely. Are they brave? No, not really, because they lack fear, their minds have been changed to eliminate it. It's almost like saying a robot or a tank is brave and heroic, it just doesn't work that way. There also is no other life available to them, their regular desires have been taken away, they have no family ties, no aspirations to live in peace and prosperity, they have nothing to lose and only know war. The Guard are the heroes because they are just regular humans with a gun and everything to lose. They face horrors that they are often not even capable of defeating. You think your Laspistol is going to take out that Carnifex? Not bloody likely. But a Guardsman must, not just because they live in a totalitarian regime that would execute them or lobotomize them if they refused, but also because if they don't everything they know will be destroyed. Their sacrifice means more because they are terrified, because they can't stand man to monster against the foe they face on their own, because there's not a lot of hope and yet they fight anyways. The Guard may be meat for the grinder, but their grim duty is made more heroic because of it, while Space Marine might be considered in spite of what they are
"Infantry is the Queen of all weapons" says an old german propaganda poster and it's truth is eternal. Let the armoured mutants do their deeds, the real fight is with boots on the ground and the countless faces of men and women of the Imperium that protect it. If all SM vanished tomorrow, the Imperium would still stand, if there was no IG there would be nobody to defend it. IG in tandem with IN are the true backbone, everything else is just cream topping. Read the Gaunt's Ghosts series for a true account of how real men fight, not tainted by armoured mutant mush-mouth propaganda.
Using Ultramarines and Imperial Fists as an example of two 'different' Space Marine chapters isn't the best example. No offense to fans of either, but they're pretty similar. Most Space Marines actually aren't Roman. You mentioned the Space Corgis and the Black Templars, but there are gallons of examples of Marines with differing cultures. Blood Angels have a renaissance-era Italy thing going on, the Raven Guard are Native American (and I don't think they or the Iron Hands are particularly obscure), White Scars are Mongolian, etc etc. Almost every First Founding Chapter has a pretty distinct personality/culture, and that's not even getting into the background Chapters like the Texas Ranger-y Iron Snakes, the Polynesian Carcharodons, or the vaguely gypsy-like Blood Ravens. Sure, they don't have the same romantization of an average guy giving up everything for the Emperor, but the Adeptus Astartes are nothing if not diverse.
