I would've said Wattson, but realised that I liked him entirely for his anime character, whereas in the games, he wasn't very defined. He had his little New Mauville sidequest, but it didn't really tell much about his character, other than he cared about having Hoenn receive power, but it could just be for selfish and economical gains :p
No, my favourite gym leader is rather Norman. Aside from being the one I enjoyed battling the most, because he was the one that really pushed me to a point of using strategy rather than just what was super effective in my most recent battle with him, I really like him conceptually as well. It's the first time we ever get to see our in-game dad, and turns out he's a gym leader! I mean, how cool is that :p But yeah, while they definitely don't really build on the want to battle him, just the fact that you get to battle your dad and prove how strong you've become on your journey in front of him, is a really good design choice, and the only thing that could really be improved, lies in the little character development that the Pokemon games really provide us with. And really, when we're presented with so many characters, it's hard to make them all fit in, although they've been starting to make quite an effort in the newer games at least :p
But yeah, I like Norman both for his concept and the design of his team, and feel like he's one of the better gym leaders the franchise has presented us with so far :p