My favorite gym leader is Drayden, even though I don't think he was a hard gym leader. The honor of most difficult I reserve for Clay, but Drayden is my favorite because he has a more likeable personality, especially his increased role in Black 2/ White 2 as he defends his city from nuclear winter and holds the key to Kyurem's transformation.
I suppose since there is often an absence of parents in the Pokemon world, fathers in particular, I love seeing characters who take on that role in a positive way, whether it is a biological father like Norman or someone like Drayden who is a father to the community.
I like his relationship with Iris, and as dragon lover myself I identify with her and can imagine Drayden being my mentor too. I also think he has the coolest gym layout of all, crawling across the body of a dragon made my dream come true.
As for a least favorite I would probably have to say Brycen. He is presented as a serious, intuitive ninja-like figure, not the most original gym leader concept but ok. However, making him also a movie star just seemed like a random detail they tacked on to make him less generic, unlike Elesa, a believable character whose work in the modelling industry defined everything about her down to her gym layout. I don't hate Brycen or any of the Unova leaders, I just don't think his character was as developed as he deserved to be, which is why he is my least favorite.