I like em all, but Frieza is best Villain.

Cell is Perfect and Cool!!!!!!!!!!!!!

I think Frieza is just a cooler villain, in general - like, he actually had plans of ruling the universe and probably would have carried that out if Goku hadn't beaten him on Namek... in fact, Frieza was the strongest around that time (until Goku went SSJ, obviously) and was basically the evil ruler of the galaxy. On the other hand, Cell was arguably smarter, but didn't really have any major plans and kind of screwed around beating the earthlings up and going for the tournament. Buu was the strongest around his time, too, but he also lacked the intelligence to really do anything ambitious, instead opting to kill Babidi and kind of screw around with the earthlings.

And his hateboner for Goku