Ashley has the highest chance of winning than any other Nintendo character.
u joking rite?
(dat pic is kewl tho)
Smash Characters who would actually stand a chance against Saitama (using their own videogame canon and not Smash canon):
Palutena: As I mentioned before, Palutena is a goddess so she could just take Saitama's strength away from him and give it to herself.
Kirby: The only thing Kirby would have to do is eat the dude and gain his powers which isn't needed because he already ate the guy. This also counts for King Dedede as he can eat things larger than himself and Meta Knight if the rumors about him and Kirby being the same species are true.
Wario: He's invincible in Wario Land II and 3 so nothing would hurt him. He could stun Saitama with his garlic breath and then just bite his head off or something. Also Shoulder Bash Master Race.
Bowser: Bowser has fallen in lava multiple times, fallen into a black hole twice and survived and went kart racing, played tennis and worked together with Peach and Mario without any awkward situations coming up. This guy is ready for anything.
Mario: Or anybody else with a invincibility granting item like the Super Star in their arsenal.
Subspace Emmisary: Taking their Smash canon into account, Wario, King Dedede, Ganondorf and Bowser could all just use their Trophy Gun to transform Saitama into a trophy.
Probably more but this is enough for now.