I like Guardians of the Covenant for being a monastic chapter where battle-brothers are not only warriors, but also scientists, philosophers, adepts of fine arts and pursuers of knowledge in general. Their red hooded cloaks on black and steel armor looks just the way I usually prefer my characters in other universes to look like.

Minotaurs for their greek-themed looks, shields and spears. The fleet-based chapter of unknown origin, shrouded in mystery. Ferocious berserkers, forever loyal and courageous. Interesting history

Grey Knights for they are doing the most important job. Grim daemonhunters, walking a thin boundary between humanity and monstrosity but never failing. To become a Grey Knight is to come through a 1000 of inhuman trials, psychic torture and mortal pain with the survival rate 1:100 out of highly talented and resilient neophytes.
They remind me of witchers from the Witcher book series. Combined with the gothic style, dark-knight demeanour.

Dark Angels for their rich controversial history

Iron Hands, because they are great artisans and warsmiths.

Space Wolves for their brawl and nordic, lupine culture

And finally and most importantly - Black Templars to whom I pledge allegiance in WH40K.
I like them for their philosophy and strong sense of loyalty, courage and brotherhood. Crusades, which I denounce and criticise in real life, I find absolutely fitting in the context of the Warhammer universe.
I like them, because they have a strong purpose and they get the job done.
They truly continue what the Emperor started: a legacy of a crusading, ever steadfast and unwavering humanity, spreading across the galaxy and cleansing it from corruption and decadence.
To me the Black Templar is the quintessence of what a warrior of the Imperium should be like.

<----Raptors Chapter: I like them because they are Reasonable Marines lol...I like the fact they are one of the few chapters that uses Camo, attack when the odds are in their favor with a talent for guerilla warfare, and allow units to act independently of the codex when necessary.

Argh i will never have war 40 k book in italian in the last six year Mondadori published only two book!!!

Since we had it better, when I start reading the 40k books, Heyne published only 1 book a year. Then they have upgraded to 3 or 4 the year and at the top they published 1 the month.
In January Black Library starts to publish the books and now we get 2 books the month. But now the books are more expensive. Nearly half of the old price

I love the Viking aspect of the space wolves' home world, and the customs brought into the legion/chapter by Russ and the fenrisians. I also love the structure and combat styles of the space wolves, and their gene-seed flaw. The heightened animalistic senses and the constant battle with the inner beast is awesome.
"It takes a lot of self control, to be this dangerous"- Longfang, Runepriest of the Vlka Fenryka, During the great crusade. (Book- "Prospero Burns.")

Why I love Raven Guard:
Specialties:
-Cool armor and colours
-Only goes for frontal assault when no other option is available, otherwise they will always go behind enemy lines.
-Infilration tactics
-Guerilla Warfare tactics
-Close-quarters engagements
-Prefered weapon Lightning Claws
"As for the leaders of the Raven Guard, they emphasize initiative and tactical skill as much or more so than personal martial prowess. The Raven Guard are much more likely to use a swift surgical strike, than to engage in long, protracted battles or sieges. However this tendency causes some tension with other Astartes Chapters who the Raven Guard consider blunt and clumsy, such as the Blood Angels."

Since I "converted" to the Sons of Guilliman:

They are in basic the same as Ultramarines (close to the Codex Astartes, similar emblem, etc.) but far less known except for Salinas and the 3rd War of Armageddon. This gives room to create an own company with an own history and to form their story, while still being well known enough to have some pre-made fluff that can be based on.

Also I love their colors, the white-blue mix is stronger than in the Ultramarines and also nobody really can harass the Sons of Guilliman as nobody knows anything about them which makes me therefore more unique.

Last but not least what I love about them is that they carry the Primarch name in the chapter to symbolize who they belong to. Kinda like the Knights Sanguine or the Sons of Dorn or well the now traitorous Sons of Horus

I've been a fan of Ultramarines since second edition. Originally I picked them as my army simply because of their colour matching those I liked or grew up supporting (local Football/Soccer team were mostly royal blue with some yellow/gold on the emblem/heraldry.)

Overtime I read more about them and the fluff surrounding them; about the First Tyrannic War and their sacrifices made to stop the Hive Fleet, how they show more concern for Imperial citizens than most Chapters do etc. and came to like them for who they were. Even as perhaps the Chapter most adherent to the Codex Astates, as would be expected given their Primach penned it, they've been known to deviate when necessary.

xvZOKvx , NikolaNesic , Brother Shrimp said it all for me.

Salamanders and Black Dragons all the way!!

Through the fires of battle, Unto the anvil of war!!
FIRE AND BONE!!

Up until a couple of years ago I had never even considered playing 40k, I'd only ever played Warhammer Fantasy.

However I started working in a school with a 40k club, I took over running the club since I knew about modelling and painting. The only space marine models they had were Space Wolves, so I started looking into their lore. As an outsider I had stereotypically seen them as just drunken Viking werewolves, but I quickly came to realise their lore is much deeper and more profound than many people give them credit for.

I proceeded to paint up the only space marine model I had (the terminator which came free with White Dwarf back when 5th edn. was released) in SW colours (yes, it had been sitting around unpainted until then. As a fantasy player I converted it with some bits from the Knights of the White Wolf). I found that I really enjoyed painting their colour scheme and that settled it for me.