I'm going to say that Ampharos definitely has to be somewhere at B, A or S rank.
Ampharos is a really good pokemon to inflict paralysis or confusion with, and next to that, it has a wide spectrum of moves and type of moves it can learn. It learns Power gem, Signal beam, Thunder, Discharge, Confuse Ray and Dragon Pulse just from leveling up! You can breed Agility on it as well to make up for its fairly low speed, but it's pretty bulky and therefore doesn't get taken down easily. Its high special attack makes up for it, and if you want to test your luck you can try to put a quick claw (is there one in the game? i forgot) on Ampharos, so it might outspeed others. It only has one weakness (ground) as well!
It's especially useful during Valarie's battle to paralyze her pokémon and then zap them to oblivion, but Ampharos is a good addition to your team in general, considering it's a good status inflicter that doesn't get knocked out that easily. It's helped me out of a pinch in more than just Valarie's battle through status inflicting, so I'd definitely recommend having it on your team.