Easiest Gym Leader you've beaten? How did you defeat him/her?
Any Rock type gym leader pretty much. Brock, Roxanne, Roark, all of them were easy as pie due to the big type advantage you could get by just selecting either a Grass or Water type starter. Now there's also the fact that they're all the first gym leaders of their respective regions, but that just adds to how easy they are, since their difficulty is greatly decreased due to it being the start of the game and stuff, and since Pokémon is also targeted at the younger players.