So there's a correct answer? It's not an opinion? If that's the case, no I can't guess. There's too many. Also do you mean main characters or main characters plus enemies and bosses and all that. Then if you have one in mind, then the chances of that are like a trillion to one.
G-Man is pretty powerful. Okay xD Nobody can kill him. >.>
sort of seconded. not sure he has a lot of physical strength/combat ability, but for being generally powerful... i'm betting he is, judging by what we've seen already anyway, ha .
Personally, I say Bowser's probably the strongest Nintendo character (or maybe even video game character).
Why?
Because while you can defeat him... nothing can seemingly ever KILL him. Black holes? No. Stars/suns? No. Lava? No. Bullets (Yoshi's Safari)? No.
I suspect you could win a lot of battles in other fictional universes by simply chucking him in somewhere, and basically watching them run out of resources/ammunition trying to even scratch the guy. Can anything top that in the long run?