I have a few favorites. I'd say my favorite overall is Jasmine. Steel types are my favorite, and I love her softspoken, timid personality. I always found it entertaining how she's apparently an eating machine, and when you see her in the restaurant she's all "Uh, all these plates are from the guy who was sitting here earlier".
Another favorite of mine is Sabrina (the game one, not the creepy anime counterpart). She's so...mysterious and introspective, yet friendly. She seems to really admire the player character.. She seems like the type of person I'd get along with really well.
And finally, I really like Skyla. Nice girl, and she was one of the more challenging gym leaders for me in Unova (the hardest was Elesa *shudder*).
The best mix of gym leaders, in my opinion, is Johto. They had a really cool mix of types, and as a whole they gave me the biggest challenge.