Mass Effect for me.
Yes yes, I know, the ME games are far from perfect. I can bang on about their flaws with the best of you, but at the end of the day it's the characters that do it for me. I'm not sure how it happened, but I've gotten attached to my assorted crew members to a higher degree than any other RPG has managed.
As for the others...
I'm pretty ambivalent about the Elder Scrolls games, but I can't deny that they're very good at what they do. I've managed to have plenty of fun with Skyrim.
I enjoyed Fallout 1 and the third game had a great setting, but they never quite managed to draw me in.
Knights of the Old Republic is one of those widely beloved games that I just don't get. The first game had an interesting and well executed twist and that one good scene when Bastilla meets her mum. I thought the entire rest of the game was terrible. Never played the second one.
No particular thought on the rest.