ABED wrote:I'd say The Red Ribbon Army as a whole is the villain. Yes, Red is at the helm, but Black kills him and takes over at the end.
I would agree there, although Blue does a lot more in a direct fashion to stand out on his own. Not to mention Tao (despite being a hired hand. ...tongue?). Silver doesn't do anything of importance in the manga but a case can be made for the anime, and while White isn't really a nice guy, he doesn't do quite enough to be remembered as a mini-arc villain.
If you were to take it as a whole, I'd say that the main villains are Piccolo, Frieza, Cell and Buu, with Vegeta as a part-timer alongside Raditz and Nappa. I do feel though that, be it GT or Super, the Pilaf Gang appears often enough that they should be considered a part of the group, even if it is through frequency than actual evil-ness. After all, without them King Piccolo wouldn't have been brought back!