Someone already said 'any Boo', but not one person said Kid Boo?
Yes, Kid Boo specifically is my favourite, and their's a good reason why. You'd probably expect me to say "Because he's the strongest villian, he roxxors! Lololl!!1" but that's only half why...
The reason why Kid Boo was the best villian (and my fave) in DBZ, is because he's Goku's complete opposite from Dragonball. Think about it; Dragonball was all about Goku as a little kid, and how he really made waves in the world because he was pure hearted and unbelievably strong. He wasn't out for conquest, wealth, or power. He just wanted to become the best he could and be a kid.
Well now Kid Boo is exactly the same...he looks, thinks, and acts just like a little kid. The only difference being is that he was pure evil, but at the same time he wasn't interested in conquest or wealth or anything like that. He just wanted to have fun (his idea of fun being mass murder and destruction...) AND, just like young Goku, he was just too damn powerful! Even at Super Saiyan 3, Goku simply could NOT overpower him!
I like to think Kid Boo was a great villian to wrap up DBZ because of that. Their's my two-cents.