I just love characters who have the blood of main villain on their hands and because I love saiyans I will mention every saiyan who does so in all of animated Dragon Ball and also the Super manga (so like, no videogames and stuff), also assists, while valuable, don't count here. 

Goku(Dragon Ball, Z, Super, GT): Unsurprisingly Goku has the most main antagonist blood in his hands, he killed Officer Black and King Piccolo in the original Dragon Ball, and Buu in DBZ, in Super/resurrection F he kills Freeza, in the Super Manga he kills Moro, and in GT he kills Baby, Super n°17 and super I Shinron at the end of GT. Of course if you count the movies and OVAs Goku killed the mains villains of the movies 'Sleeping Princess in Devil's castle', 'Dead Zone', 'The world's strongest', 'The tree of might', 'Lord Slug', 'The return of Cooler' and 'Wrath of the Dragon', I might have forgotten one or two but you get the gist. Again, assist doesn't count so no Broly or Janemba by the technicality that he's half of the saiyan who killed him.

Gohan (Dragon Ball Z/Super): Gohan has less trophies to his name but he still has quite a few. Of course he killed Cell in the Cell saga, Bojack and Broly in their respective movies, as well as Cell Max in Super Hero. 

Bardock(Dragon Ball episode of Bardock): Bardock only has one such kill to his name but he still has one so it's only natural I would mention it. Bardock kills Chilled, the main villain of the OVA Episode of Bardock

Goten and Trunks (Dragon Ball Z: Bio Broly): These two are like the forgotten wonder children of Dragon Ball but like Bardock they do have at least one main villain head to their name and that is Bio Broly from the DBZ movie of the same name. 

Gogeta(Dragon Ball Z fusion Reborn): And finally, Gogeta, the fusion made from Goku kills Janemba in Dragon Ball Z's twelfth movie "Fusion Reborn". 

Well that was a bit long of me to gather that list but I figure I owe it to all the saiyans who successfully killed the main villain of any story to be as thorough as possible. You can say proudly say there's not a single jobber on this list.

Characters who successfully killed the main villain of a story