Probably Vegeta. Vegeta went through many weird phases. He was evil, then he teamed up with the good guys because they had a common enemy in Freeza, then he chills with the good guys after that and it's kind of ambiguous what exactly he's up to since he seemingly still wants to kill Goku but is banging the likes of Bulma. I think at this point he's not sure what he wants to do so he's just going with the flow, he's still evil but he's also alone and depressed at being bested by Goku all the time. At this point he's still all about himself, but even he can't escape the need for some companionship. Skipping ahead, way later when he sees Trunks die it probably hits him all at once like, "...that's my son," and then his character progression is mostly finished with that realization.
I think the Boo arc did an awful job with Vegeta, turning him evil again the way he did, but regardless of my opinion, the Boo arc would be where the last few shreds of Vegeta's evil go away I guess.
Piccolo's development on the other hand was quite rapid. He trained Gohan to beat a common enemy, was treated as a friend by Gohan, and then the goodness in his heart prevailed since he was touched by this. I liked it, but that's all there was to it.