It's kind of a giant list but as of right now it would be:
Elder Scrolls IV: Oblivion (for it massive open world and gameplay)
Star Ocean: Till the End of Time (best star ocean game ever with a great storyline and amazing gameplay)
Final Fantasy X (good gameplay and allowing any character to eventually excel in everything and be overpowered is fun, especially having Yuna do Armor Break and hitting high physical damage. Also for the chocobo theme)
Demon's Souls: (difficult to play and master but is rewarding in the end, has good gameplay and co-op)
Phantasy Star Portable 2: (good gameplay and probably the best phantasy star game to-date. Also is a good PSP RPG)
Fallout 3: (good gameplay, lots of decision making and plenty of things to do, even if you beat the game and completed everything and it has a good replay value)
Fallout New Vegas: (more of an "updated" Fallout 3... except that what you kill in vegas, stays in vegas...)
Borderlands: (Lots of looting, questing, and being annoyed by the "everyone hates you nor cares about you" claptraps)
I don't have a #1 favorite because theres too many good RPGs out there.