1. Buggy: Not only is he my favorite villain, but he's one of my favorite characters. He's just hilarious, he has an awesome power, and is one of those persistant, bumbling characters. I like.
2. Arlong: He's just completely badass. Not only that, but I also love his sense of style (hawaiian shirts.. <3), and the way he fights. Seeing him and Luffy tear the hell out of Arlong Park in their fight was great.
3. Foxy: Another great comedic villain! His devil fruit power is one of my favorites, and he found a lot of tricky and creative ways to use it. He's hilarious; the fight between him and Luffy is one of my alltime favorites.
4. Smoker: Just an all-around great character.
5. Wapol: He was pretty funny as well, and I loved the Baku Baku fruit. But he was a pretty big jerk.
6. Crocodile: A really good villain; possibly even more heartless than Arlong! But he got on my nerves after a while (but I've discussed that enough).
7. Ener: His Goro Goro powers were cool, but I got tired of him pretty quickly. He seemed really childless, and was just an asshole. And he was so friggin' cheap! I literally screamed "BULLSHIT!" when he brought himself back to life after Wiper owned him. I just don't think he was a very appropriate villain for a dramatic arc like Skypiea. But he was alright.
8. Kuro: He's just an asshole. But, he has a cool fighting style/weapons, and was cool enough to have someone awesome like Jango on board.
9. CP9: Ugh, traitorous assholes. They do have an awesome (but unbelievably cheap) fighting style, and I'm a fan of Blueno.
10. Don Krieg: I've gone over this before..
I didn't count minions (villains that had their own arcs but were just following orders, like Mr. 3 or Mr. 5), sub-villains (like Hatchan or Gedatsu), characters that were antagonists at first but became allies (Wiper, Franky), and losers (Bellamy). I didn't want to make the list too long.