I know nothing of Warhammer and too little of Star Trek to chose either of those. Based on what I know of Mass Effect and, Star Wars I think I'd have to chose Star Wars. If I'm not a Jedi, there is a chance that I will be brutally murdered during a war or, by a planet-destroying super weapon. It would be cool being a Twi-Lek or, Wookiee though.
I would love to be a part of the Mass Effect reality too but, there are too many variables there. Perhaps I'm living in the renegade Shepard universe: I start out living in a Galaxy where the other aliens treat me like a second class citizen but once Shepard kills off the council, everyone singles me out and treat me worse because some military douche is making the rest of humans look worse than the Krogan. Meanwhile there's the risk of a Reaper invasion and, the other risk of randomly being enslaved or, captured by Batarians, Vorcha, Krogans or, other humans.