Of course, the first response I have is that there is no best race for each class. I mean, let's look at shaman for a second; an Alliance shaman can only be a draenei or a dwarf. Now, I personally think that a draenei shaman is just right somehow -- I really can't explain it -- and it was a really big decision for me to switch my shaman from a draenei to a dwarf. I did it for Stoneform, OK?
To me, paladins should be human. Warlocks should be undead. Rogues? Hmm, worgen, I think. You'll note how there is absolutely no justification whatsoever for these choices -- it's just a feeling that these are the best races for these classes. They're the races that fit, somehow, absolutely and completely personal to me and not relating to anyone else's point of view! There's no logic to it.
Orcs should be warriors. They're strong, powerful-looking characters; an orc mage or warlock would just be wrong somehow. Again, this is deeply personal, and I wouldn't have any issue with someone who felt an orc warlock was a perfect class/race combo!
What do you think? Am I spouting utter nonsense? (Wouldn't be the first time.) What do you think are great race/class combos, and which are wrong -- or worse, creepy?