Going to go with the popular choice and say Mass Effect.
I don't understand why people keep mentioning things like 40k or various fantasy worlds. Do you realise how likely you'd be to die very, very young? Mass Effect has better technology, better agriculture, better medicine, better everything. Assuming we're not talking about the Reaping time period (which we'll know virtually nothing about until the next game is out anyway), you're probably looking at a pretty long, pretty ideal life.
As for role, an average person on pretty much any of the major worlds would be awesome. Obviously being a Spectre would be okay too...(again, given that we're not talking about the time period of the games where Spectres seem to be dropping like flies).