I love gym leaders. Some look pretty cool, and others are pretty challenging. There are many Gym leaders that I like:
I love Misty, Janine (better than Koga), Sabrina, Morty (I love its design), Clair (pretty challenging), Gardenia, Maylene, Fantina, Candice, Volkner, Elesa (From BW2, it has a better design IMO), Brycen and Marlon.
WOWW, this is a huge list. You can see many generation 4 gym leaders in there .... that's because I love 4th gen. But, if i had to pick one of them, it would be .... Gardenia. Idk, it's just because I spent many time battling with her (I was a noob), and I love her design so ... yeah!