Well, my tastes are pretty varied. Most of what has been posted is worth a look....
The hype machine promoting Mass Effect was one of the reasons I got a 360 after skipping the previous generation of consoles altogether. It turned out that it fell short on many features (such as dynamic interaction between characters), so it was something of a disappointment--it simply wasn't revolutionary in any respect--but it's still a rock-solid RPG with FPS elements.
Fallout 3 and Oblivion are different spins on the same kind of game. They're exceptional in the sheer volume of content that they have for the player to discover. They really take the open-world concept in a direction few others go. Mass Effect's developers could stand to take a note or two from Bethesda.
Marvel Ultimate Alliance is a great superhero-genre RPG.