One thing I don't like about Oblivion is the Dark Elf male voice. (Provided by Craig Sechler, who is brilliantly creepy and annoying in a good way as the Adoring Fan.) The voice acting itself is good; it just lacks the dark, grating quality that really made Dark Elves for me in Morrowind.
Except for that one minor point, though, I like the voice acting. It's good, it's interesting to listen to, the writing's good. My favorite voice in the game (excluding characters with specific voices) is the Redguard male voice, by Michael Mack. He does some of my favorite main-ish characters, Owyn and Baurus. Owyn's just cool, and Baurus is the only main quest secondary character who doesn't at some point become irritating. (I'm looking at you, Jauffre.) And I just generally find it the most interesting voice; whenever multiple races have the same rumor script, I'll typically like the Redguard version best.
Also a plus: after you finish the main quest, Redguards don't sing at you.