Probably Blue just pipping Chuck and Blaine to the top spot. Blaine's got some decent pokémon, and the fact that his old gym is now a volcano (frickin' cool), his new gym is an icy island that he shares with a legendary pokémon (literally cool) and he may/may not have had a hand in the creation of Mewtwo (still badass despite being over 15 years old).
Chuck meanwhile, works out, likes waterfalls and has a Poliwrath - possibly my favourite fighting-type pokémon alongside Lucario and Gallade. He also has a pretty sweet location for a gym (even if he doesn't keep a close enough eye on the town to stop Silver stealing a Sneasel).
But Blue is the opponent from LeafGreen which gives added happiness from defeating him, he has Giovanni's former gym in a town with class background music (badass yet also relaxing), and his pokémon are pretty cool - although Exeggutor is nowhere near being a favourite, it's pretty tough. And I love all his others, especially Rhyhorn.