Ech. So many to choose from. I've always been drawn to the Stunties for some reason, and Chaos as well, so that makes Chaos Dwarfs a very strong contender. Of course, I always thought the hats were ridiculous, but with the redesign that accompanied the Hellcannon (not that Chaos proper should be allowed to have artillery, but c'est la vie), they've chiseled and burned a place in my heart.
Tomb Kings are another one, particularly as I've always been somewhat enamoured with the ancient Egyptians. And for similar reasons, the Brets have a good case for making a claim to the title - I probably knew who King Arthur was before I was aware that Canada wasn't a monarchy.
For all that, though, I'd probably have to say Ogre Kingdoms. I saw the previews, and immediately knew that I had to have them (this was, of course, before anyone had worked out what a shite list they were saddled with). Not only have they got some great minis (some truly awful ones too, but that's beside the point), but the fluff is fantastic. There are days I wish I was an Ogre. Their lives are so beautifully simple - you fight, you eat (generally what you just fought), and you find something else to fight. A lot like Orcs, really, but with more feasting involved. And how many Orcs can say they wrestled a giant to the ground? Or scaled the biggest mountain in the Old World, and ate a bit of it? Or saw, in person, their god? They can live through practically anything, they don't have particularly complex emotions, they don't feel the need to waste time working out the right and wrong of things...
Gods, I wish I was an Ogre.
Four Colditz salads and a Hermann Goering.