Buu is an obvious choice, every loves Buu. As for fusion, i think in an episode where they mention it only works when the two are similar in size, age and power level, thats why Gohan and Vegeta didn't fuse. Dunno why Goten and Trunks didn't. I agree Gotenk is one of the coolest. He's # 1 on my list!
They should show more of him.