The best is to paint however you feel like.
Whichever seems cool to you.
Dark Angels are secretive Knights.
Blood Angels are beautiful engraged space vampires.
Space Wolves are space Vikings with some werewolves and wolf mounts.
Ultramarines are the Greko-Roman poster-boys.
Imperial Fists like to build fortified positions.
White Scars are Space Mongolians who ride bikes.
Iron Hands love machines and seek to become machines.
Black Templars are Crusaders.
Salamanders have coal black skin and red eyes and love fire and smithing.
Raven Guard like stealth and jump packs.
Those are some of the main Chapters.
The setting supports you making your own Chapters should you wish. Of course meme-Chapters only get you so far. Lol