Cloud is indead stronger than Sephiroth. In both the Game and advant Children they Pictate Cloud as stronger by pitting them in a one on one fight. Strength in a fight is more than shear power, speed, or specific powerful skills. Its also about cunning and battle sense. There isn't much of a debate of who's stronger, as Cloud came out Victorious twice.

That said Sephiroth is only a decent Villian, Jenova better, but not by much.

A real villian my friend is Kefka or Kuja.