Dishonored and Fable, can't decide between them. Dishonored - Even if it was quite short, it's still my favorite RPG. It simply is one of my favorites because the main character, Corvo Attano, is such a well done and a guy full of mysteries that you simply fall in love with him.
Fable - The first one, The Lost Chapters.. I think there wasn't other game that I played so many times. I literally finished it 14 times. 14! And I still feel the urge to play it. What got me? The story and the way it twists and the main villain, Jack of Blades. How can you not like that guy?
My favorites would have to be Lunar and Lunar 2, the Bard's Tale Trilogy, Final Fantasy 1, 4, 6, and the Shining series (Shining in the Darkness, Shining Force 1, 2, 3, Shining the Holy Ark, Shining Tears, Shining Force Neo, etc.).
Although a lot of good RPG's were mentioned in this thread, I'd like to point out that other than Zelda 2 : The Adventures of Link, no Legend of Zelda game is an RPG.
Earthbound, Chrono trigger, pokemon (any), Mario and Green guy: Superstar saga, and FFX.
Earthbound - Deep story, great mechanics, and lovable characters.
Chrono trigger - Incredibly deep story, love the uniqueness of the battles, and the uniqueness of the story (at the time)
Pokemon - 'nuff said.
Mario and green guy: Superstar saga - Hilarious, love the timed hits mechanic, love the ridiculousness of it all.
FFX - After three years since I've played it... I'm still welling up in tears when I hear the main theme, To Zanarkend
most of my favs have been mentioned here. the baldurs gate games, the icewind dale games all took hours upon hours from my days and nights. Same with the Diablo series. The xcom games, old and new. there were number of old NES games that i enjoyed too. Can i pick a favorite single game. Prob not. Though the original xcom might be the one. I regularly went back to that over the course of 15 years. Think i might dive into Dragon Age Inquisition tonight though