Pantheon is specifically a lane-bully counter.

Since building defense is the worst option now unless you have Galio's or Gragas's Q, people have to pick a champion that can actually handle the lane bullies-- many of whom rely on one or two AA's in their early trades. And Pantheon's there when the only two viable tanks are banned or not your forte.

He's not overpowered. He's the scissors to your all-encompassing paper now that rocks are pebbles.