This is the big one, the one that gets compared to Star Wars. Mass Effect is one of the greatest RPG experiences ever etched on to a disc, one that its sequel didn't quite match.
With Mass Effect not only did you get a whole world to explore, you got a whole galaxy to rummage around in. But it was more Mass Effect was most impressive because of what went on right in front of your face, rather than thousands of light years away.
It was the personal relationships you formed and the danger of stretching the professional ones to breaking point. It was the subtle mechanics of the conversation system and the fluidity of dialogue coming from the perfectly defined characters.
The biggest triumph of Bioware's epic, genre giant, however, was its willingness and ability to place a phenomenal amount of control over a fantastic linear story into the players' hands.
Mass Effect 2 upped the action of the series and was roundly praised for almost every aspect of its experience. As far as role-playing was concerned, however, it did lose out a little bit in the sequel and so, for now, there's only really one true way to quench a thirst for some intergalactic RPGing.