Okay, this took a little longer than I thought to complete, thanks to an absolutely insane schedule last week. However, I'm back now and ready to reveal the #1 Reformed Villain!
1. Vegeta - Probably my favourite ongoing storyline thoughout Dragon Ball Z and GT is that of Vegeta. When we first meet him, he is evil incarnate; he kills his own partner, Nappa, because he disobeyed an order. Later, on Planet Namek, Vegeta is forced to work together with the heroes to stop a greater evil, but he makes it very clear he is still serving his own agenda. Vegeta is humbled by Frieza, and finds himself on Earth with the rest of the survivors from Planet Namek.
All throughout the series, with the exception of his initial appearance, Vegeta has lagged behind Goku in power but has been close enough to keep it close. By the end of the series though, Vegeta accepts that Goku is the more powerful, and the two work together to defeat Buu. It really was fun to watch these two finally, truly work together, instead of their old "Stand aside, I want to fight him!" approach to "teamwork".
The best villain reformations are ones that are done gradually over time, and Vegeta's was a slow burn that evolved organically that lead to a satisfying conclusion. The story of Vegeta is, I think, the best part of Dragon Ball Z and GT. Sorry Goku!
That's it, that's the list! There are some omissions that may seem criminal (you have Harmony and not Spike or Angel?!!), but later this week I'll lay out why I disqualified certain characters from making the list. That being said, are there characters you think I missed?