Considering Sephiroth is just a panzy momma's boy? Just about anyone. I mean, seriously, the only time you *ever* fight Sephiroth without Jenova's backup.... he can be killed with a single hit from a counter-attack materia.
Now, you want a scary biochemical monster- Parasite Eve. THAT thing would have bent Jenova over and used her as its personal plaything.
Almost any X-man character would destroy Sephiroth. In fact, pretty much any comic hero that isn't purely human, and even then most of them could with a decent team.
I'm pretty sure a team of white mages from FF1 could bring him down.
