Gotenks

One of the most powerful characters in all of DBZ, Gotenks is the fusion result of Trunks and Goten. Goku and Piccolo teach Goten and Trunks the fusion dance, a technique used to become a warrior many times more powerful than either person performing the dance. Gotenks is a completely different person, and so his personality is totally different than either boy who fused to create him. Gotenks' power is fantastic, and he is one of only two characters that have ever achieved SSJ3. He fights Majin Buu twice. The first time Gotenks loses pitifully, The 2nd time they face off is the famous battle which begins in the Time Chamber, then expands to Kami's Palace (which is completely demolished) and finally ends on Earth. At SSJ3, Gotenks can easily defeat Buu, and he knows it. But instead of taking him out, Gotenks' cockiness gets the better of him and he toys with Buu. Unfortunately, he defuses before he can destroy Buu. We almost see Gotenks in GT but Goku says it no use to fuse because of how much stronger the enemies are. Instead Goku just asks for them to hand over their energy.