So, after a number of people posted votes on my blog for my armies of destruction, it seems that there are two distinct choices.
For 40k, it seems that Tyranid are the most popular choice. I'm inclined to take their advice. Tyranid have always had a very strange attraction for me. Who doesn't like hordes of infantry supported by giant crushing monsters? In terms of strategy, I will likely use more big bugs than the tiny ones. Oh, and an assload of guns.
For Fantasy, I'm not entirely sure about the Warriors of Chaos choice. I'm thinking a horde of Khorne sounds like a ton of fun. Armored Berserkers of Doom! Dark Elves are a close second in my head...
I'll have to think a little more on this topic.