Vegeta is one of the most well-known of the anime villains who redeem themselves and become heroes, but he actually isn't the only character from the Dragon Ball universe to make such a transition. In fact, the majority of Goku's friends were once people who directly opposed him in some way.
Some Dragon Ball heroes who started as villains, like Vegeta and Piccolo, have well-known arcs with memorable transitions from villainy to heroism. Their motives for bad deeds are understandable, and their reasons for changing make sense too. Others, like Oolong or the Pilaf Gang, have motivations that aren't as well explored but are still entertaining.
Which bad guy turned good is your favorite?