for rpg, I'm gonna have to go with both oblivion and mass effect.
omg I can't wait for skyrim either. already pre-ordered it ^^
call of duty and halo
IMO Star wars battlefront 2
Twilight princess and super mario galaxy 1 and 2. :)
Best according to who? Everyone has different taste, dude. Personally I think the best RPG was the Fallout series.
I was always a fan of KOTOR, and, to a lesser extent, the original Mass Effect. I like the "space cop" feel of both games, and they had greatly atmospheric music.
Out of RPG games, personally my favorite is Final Fantasy 6. Out of every game ever made, I like a lot, such as Half-Life, TimeSplitters, Metal Gear, etc. However, this answer is merely subjective.
Since you asked about RPGs, I'll exclude MMORPGs, like WoW. Thus when talking about the best RPGs Diablo 1&2, Oblivion and Dragon Age: Origins come to my mind. I'd probably pick Oblivion for its fantastic (and terribly long) gameplay and the fun I had. However note should be made that Elder Scrolls V: Skyrim are to be published soon. If MMORPGs were to be included, then I'd pick WoW, hands down.