The Animorphs' strengths are a) ability to put others into a trance if they can touch them (in most cases), b) turn into any animal they can touch, c) a large bank of animal morphs to choose from. And that's not counting their individual skills (e.g. Ax's tech knowledge) or their allies (like the Chee).
I can't really see them winning against any superhero in direct combat, but there are other ways they could win. For example, I'm sure Tobias would have a breeze figuring out Superman's secret identity. The Anis could sneak into his house, they take turns acquiring him while he's sleeping, and in the meantime Cassie morphs into Yeerk, gets inside his head and learns everything about him. By the time they're done, the Animorphs can defeat Superman by threatening to reveal his secret identity, beating him up in Superman morph, forcing him to do damage to himself via Yeerk control… it's not even fair.