Sabrina the Psychic Gym Leader of Kanto's Saffron City FTW!
For one, she was able to outclass a rival gym, the Fighting Dojo (yeah, I know about type advantages, but they could have used Blaziken, Infernape, and Lucario to help!).
Secondly, I heard she was quite the powerhouse in the RBY days (I started Pokemon games in Gen. III with Sapphire).
Thirdly, she dislikes fighting but is still a force to be reckoned with. Now that's uber.
And finally, FR/LG just made her look freaking hot (anime version in Ash's Gym Battle looked creepy).
Second is probably tied between Brock and Misty, for numerous reasons in the games and anime. Mostly due to Brock's know-how and Misty's beauty. And third goes to Koga, because PokeNinjas rule!!