What I look for in an RPG is the ability to create a character that feels real, and then have interactions that feel real, all while doing something heroic and fun and creative in the process. Basically, I want to have a fantasy experience with feedback from outside my own head, in a way that makes sense. What I don't like is the oversimplification found in many games these days (D&D 4e is a great example), simply because the gaming experience that provides is something I can usually get from a video game anyway.
So far, I think my favorite game is Shadowrun 3e. It's balanced enough that you can create any type of character you want and not worry too much about whether you're going to be too strong or too weak... your power is based soley on your ability to optimize a bit, not in any way on what kind of character you wanted to make, and the same amount of work creates the same power level. Plus, the power level difference in optimizing vs not optimizing is significantly less than the power level difference between playing creatively and intelligently and playing stupidly, so it really does come down to how well you play, not how many books you have or what the DM allows in creation. And, unlike most other well balanced RPGs, you can still do all kinds of crazy stuff and make all kinds of characters... I'd say there's more versitility in character creation in Shadowrun than in any other game out there. I can't think of a single D&D character that I can't make a Shadowrun version of, nor any fantasy character (though the power level may be lower, you can't make superman at full power for obvious reasons). And then the characters you make can do cool and creative things.
I do like D&D as well though, simply because while the balance issues are obvious, you can still make all kinds of characters, and that's really important. Plus, the characters can do cool things, which is sorely missing in 4e.
JaronK