It seems a lot of people have picked Buu, and I have to say, they're probably right, if we're staying canon. As far as Frieza goes, if you're good, he'll enslave you until you're no longer useful (which may be a pretty short time depending on who you are). And Cell will at least play with you a little bit. With Buu, unless you happen to be one of the heroes, then you're just dead period. Unless you happen to have a knack for not staying dead or a specifically convenient plot device, or you happen to have surpassed the gods already. By the time Goku got to Buu it was really getting kind of silly how powerful everyone was.