Well, you can combine both ideas. Have them help each other defeat the villains, but then fight each other to prove who's better.

I personally go with Giga Bowser. He could just step on most of them. He's not fast though. Even slower than Bowser. So, he can use his might to shake the ground and keep doing so until the ground crumbles away. Of course, he could just stand there. He's not bright though, so he may simply charge one of the characters.

Yaridovich is intelligent, quite clever. He could use his smarts to find a way to hide until one person is left standing.

Belome could eat Booster. Booster clone. Belome could eat Yaridovich. Yaridovich clone. That might be a bad thing for all in the tournament except Belome.