Tough question. Buu, in all his various forms, was an interesting villain. Broly, was really cool too, at least in the first movie with him - Less a fan of "BioBroly" really. Cell was awesome, and quite a dramatic arc. Frieza is so very iconic, down to even being the subject of the new stuff in question.
I think I'll have to go with Buu though. The various different personalities he took on in the various forms were both entertaining and powerful, so I think I like him most as a villain - though I prefer the Cell arc the most still, for the story aspects.