Freeza, just because he is a bonafide VILLAIN, and has the most awesome backstory in any part of the series, that with all the retcon basically spans the Goku's life wether he is around or not. And all those transformations, it wasn't so annoying until Cell and Buu starting shapeshifting repeatedly. He also owned Kuririn twice.
Baby, being one of the small number of elements of GT that I really like, a villain with what I believe to be unrealised potential, mainly due to a kind of watered down plot. Same as Freeza in that his story kind of spans the mythos in more than a "been around for millions of years but only decided to show up now that we can stand up to him" way.
The Freeza and Baby argument could also be applied to Cell and Li Shenron I guess, but I just prefer the above characters.




