Kanto has the highest proportion of really good Gym Leaders out of any region, to be honest. Even the first couple are particularly memorable characters (granted, that may partly due to nostalgia and their presence in the anime... but they're still memorable nonetheless.) I mean, come on. Who really remembers Falkner or that other one that was first in RSE? (Can't even remember his/her name...)
But if I had to choose one, it'd be Sabrina. I can still remember her Gym challenge in both the games and the anime, back when I still watched it. She was crazy-powerful, had a personality, and her Gym was a beast to get through. All of these are definitive plus points in my book, so she gets my vote.