Geno Whirl does 9999 damage if you time it correctly, so that's the strongest attack in the game (doesn't work on bosses). Mario's lazy shell is the strongest weapon for your basic attack in the game, but it depends on Mario's stats, too- so it doesn't necessarily do the most damage, but it has the -potential- to do the most damage (ie, if you never boost Mario's attack stat once when he levels, but you do it all the time for say, Bowser, Bowser will do more damage with his final weapon. I did this the first time I played SMRPG and Mario did like, a third of Bowser's damage).
I think if you unequipped everyone's gear and didn't take into account the stat bonus when your characters leveled, it would more or work like this:
Mario would be all-around, Geno would be the fastest (weak physical defense), Bowser would have the best physical attack and defense (weak Magic) , Mallow would have the best Magic attack and defense(?) (weak physical attack), and Peach would have the best something-or-other-I'm-not-sure. Best healerness, there you go.
I think that's generally how it breaks down, anyway. I might've made a mistake or two.