Charizard: 1: Give him hurricane so he isn't completely crippled by rain 2. Give him a Sp. Atk version of Dragon Dance 3. Remove that stupid HP loss part of Solar Power. It's not overpowered at all. Chlorophyll gives twice the boost with no damage.
Alternatively (But this would work for any pokemon with remotely decent attacking stats): 1. Give him Speed Boost 2. Give him Nasty Plot 3. Voila, you've got yourself an Uber Charizard on your hands.
Charizard has lower attacking stats than blaziken, much worse coverage with two fairly redundant stabs, and a quad stealth rock weakness. I'd bet that'd be perfectly handable in OU.