Well, having recently returned to 40k after many years away I only have one force at present, so I would be playing Marines. At present I consider them a home brew chapter that uses a combination of Red Scorpions and Astral Claws mostly (as the characters match my fluff). Mostly Claws though, as the fluff of the force is an Astral Claws successor chapter.

However, what I really love is Ravenguard, and my force is modeled with their iconography. If they are more viable in the soon-coming new SM codex I might switch and play as RG.