Well, in terms of believable redemption, I put Vegita on par with Tousen, some of the espada, or Sesshomaru when compared to other villains in their respective universes. I'm also looking at the length at which they appeared to be the main antagonists.
I would put Aizen on the same level as Freeza, Cell, kid Buu (as DBZ Sagas were not really connected), or Naraku. There is just no redemption on his part that would allow him to join forces with Ichigo in the way some type of Antihero. It would be more like transformers when Megatron teams up with Optimus, cause we all know how that will end up.