Kanto
Favorite: Blaine (Can't help but love his fire puns. Also love how even when his gym got destroyed by a Volcano, he resorts to using some random cave for a gym)
Least Favorite: Erika (if you've ever seen "Destiny Red" from the Nuzlocke comics, it will explain why it's my least favorite gym)
Johto
Favorite: Morty (Even though he killed my starter in my Heartgold nuzlocke, the character himself interested me the most, also his overworld sprite in gen 2 reminds me of Snake from Metal Gear)
Least Favorite: Pryce (Was just there, barely remember him)
Hoenn
Favorite: Norman (Oh my god we actually have a father that is also a gym leader )
Least Favorite: Tate and Liza (they only use two pokemon......just....very disappointing, although they did fix that in emerald but then made them have only two pokemon again in ORAS)
Sinnoh
Favorite: Volkner ( I kinda been through the same thing the guy was going through, the whole losing motivation to do stuff so I got interested in him the most)
Least Favorite: Byron (Even though its cool that hes Roark's father, I tend to forget about his existence)
Unova
Favorite: Elesa (Her character, and her gym was really cool, especially her gym in Black2/White2. Also, she loves bad puns XD)
Least Favorite: Marlon (The only Unova gym leader that I just didn't care about. I just didn't like him)
Kalos
Favorite: Wulfric (The talk about the whole "I could be just a pushover or be your worst gym leader" talk gave me a bit of a laugh, especially after 3 minutes of completely destroying his team)
Least Favorite: Olympia (The most forgettable, especially since her gym was a breeze, like most of the Kalos gyms)
Reborn
Favorite: Radomus (Loved his gym puzzle, his character, and hes a pretty fancy guy that loves to pick the right outfits for any situation)
Least Favorite: Noel (I swear, everytime I get to him, I always end up doing a lot of training before I beat him. I usually get a couple of new pokemon and train them up when I get to him. Also, that Clefable >.>'')