John Cena always kicks out at 2. His opponent needs help form Brock Lesnar to win, even then it would take a curbstomp to put down the Dr. of Thuganomics.
Kirby can crack planets (in Super Star and Amazing Mirror), but cannot absorb enemies stronger than him (like bosses). His feats are mostly inconsistent and used for laughs in his universe. Also, his reaction and strength are shown in the sub games "samurai kirby" and "quick draw", where he is mostly on par with regular enemies and bosses.
Buu is way too fast, strong and has better attacks!