The original Ultima series...not UO, I've never played that. Ultima 7 (with 7.5) is one of the best RPGs I've ever played. 5, 4, and 6, in order of my love for them, come next.
I greatly enjoyed the Magic Candle series, although was only able to beat 1 and 2 (3 had some horrible corruption bugs that never were fixed, as far as I know, although it might have been my crappy computer at the time). I'd love to play those through again if anyone knows of any emulators for them...
I played almost all of the Might and Magic series, except for 3, 8, and 9, and I would probably say that 4-5 (the Xeen pair) were my favorites. Again, I'd love to play these again, emulated, I think I still have the disks sitting around somewhere.
I only played a few Wizardry and Bard's Tale games, but they were fun.
I also had fun with some of the SSI AD&D Gold Box games, especially the high level ones like Pools of Darkness and Dark Queen of Krynn. Treasures of the Savage Frontier was fun too.
I enjoyed Arena and Daggerfall but never beat them; Morrowind, however, I played all the way through and enjoyed very much. I'd probably have started a new character and played through again if it weren't for EQ becoming my gaming focus.
I've enjoyed many others, including on consoles, but those are the main computer ones I can remember. Man, one of these days I need to take an old computer and set it up to run DOS so I can relive some great memories.
Game on,
The Oneiromancer