My favorite gym leader is Maylene. Her character was nice and humble and she did the fighting type proud. Her gym design wasn't exactly the best, I'll admit, but she stood out too me. Runner up goes to Byron, but only the Anime version.
I haven't played the games, so this is based just one what I've seen in the anime, but my favorite gym leader was Byron. His character design is awesome. I love how both him and his son are gym leaders of two different gyms. And, I'm going to do my best to not sound bitter here, he's one of the few Sinnoh gym leaders in the anime who actually takes his responsibilities as gym leader seriously (Please don't get me started on Fantina or Candice, as well as Maylene and Volkner to a lesser extent).
Volkner for sure. I think he really sets a serious mood at the end of the game. He is a sneak peek at some of the harder trainers in the game, and I also liked him growing up. His Electivire led to me wanting my first Electivire (since I hadn't seen one before that).
Roark for being cute lol. I also like Candice for her adorable design! Overall Sinnoh had the most forgettable gym leaders for me, but those two are cool. :heart:
Byron, at least in the anime. I love large hams, and they don't come much larger or hammier than screaming "I LOVE [insert object of affection here]!" every time something happens. Complete with shovel wielding and explosions. Hell yes. xD; I LOVE FOSSILS! AND I LOVE BASTIODON TOO! *explosion*
Candice. I love ice types and I don't know if I love this character, but I like her design and really love the ice typing. I didn't really enjoy her gym but I felt like it wasn't the worst. I more like her ice typing rather than anything else.
I've always liked Volkner, probably because he had a Luxray which -at the time- was one of my favourite Pokémon. He's the most memorable Sinnoh gym leader too imo.
Fantina struck me as much more interesting than the other gym leaders because 1) I had to travel for such a hecking long time before I could challenge her gym! and 2) she’s at least bilingual or a foreigner from her accent. I have a lot of respect for people who can speak more than one language/people who move to a different country despite knowing there will be a big culture change. I moved once to a new country and it’s not for me, but seeing another foreigner like Fantina in the games spoke to me. Oh, and Candice because she’s super adorable, in the games, and the anime!
Roark because I have always enjoyed the fact that he is the only Gym Leader with a father at another Gym as the leader but he stays with the original theme of Rock whereas Byron went to Steel. still a classic rock type so I had to put it in. Then as a tribute to my second favorite Sinnoh gym leader