TG station is good for a quick more or less 'vanilla' round. Colonial Marines is good for those who have an itch for action. Although, I do really enjoy the predator players; they make the round quite exciting.
servers wise i feel like it goes as:
TG is the bog-standard, most servers are based on its code so it doesn't deviate too much.
Colonial Marines is if you want action but rather have friendly fire caused casualties.
Corporate Mercenaries if you want nothing but memes and shooting.
Goon for bees and and a glorified water park.
Bay12 if you like the more serious rp but don't mind occasional references to dwarf fortress.
Hippie if you don't give a shit and want to spam your scream as a skeleton and convert engineering into the radiation nation.
Theres more but then it starts to deviate a bit too much or has a very negative history on it such as lebenstraum