My least favorite are Bosmer. Something about them just strikes me as annoying.
Orcs are pretty close as well, at least in the Oblivion version of them. The voices and diction seem off to me, and I think if they'd just made them be something other than green it wouldn't be as bad.
Playing with the skin color sliders can make a blue orc, or other various colors.
And I personally love being a Bosmer. Being small means you are a smaller target. Some say they are ugly, but aren't almost all NPCs in oblivion ugly to be honest. I have a good looking Bosmer I made without using any mods. Also, you shouldn't hate a whole race due to 2 NPCs. That is called globalization and is plain wrong. Would you say the same thing about a whole race in real life if just one person of that race bothered you?
I also would go as far to say that there are probably more bosmer lovers than haters playing oblivion, and I believe they are one of the more popular races. Especially when I think about how from my experience with various polls I have found that more people play sneaky archers than people play warriors or mages. Bosmer is the perfect race for archers imho.