Depends on the setting. If we're playing Earthdawn, I very well might say Orks are my favourite race, but if it were Eberron, I'd say they were my least favourite.
But in "core" D&D? Humans probably wind up being my favourite race, simply because of the versatility inherent in the race. You can do a lot with it. Half-Elves come a very close second.
As for most hated race? Probably gnomes, actually. Mostly because they just seem silly, and every time I see one of the poxy blighters, it's just some idiot who wants his character to be "funny". Those who want to play serious "little people" stick to halflings, which are so much better.