Hmm.... My favorite of all Gym Leader is probably Skyla. I love her design and flying was once my favorite types. It is still up there, but not as high as it once was. I also really lover her name.
As for each region.
Kanto - Giovanni.
This is largely due to the manga. They made him a much better character and he was so powerful compared to everyone else. His fight between Red was insane and all the lore in the manga just made him a wonderful character compared to the games and anime.
Johto - Clair
She trained with Lance and is a Dragon Master, but her design is amazing. But that is about it. Overall, while I love Johto, I really don't care about any of the Gym Leaders there. None of them where very memorable in my opinion
Hoenn - Norman/Brawly.
I love Brawly's style. But Norman edges him out just barely. Not only is he your father in the game, they also made him a great character in the manga. And he is also really wonderful in the anime.
Sinnoh - Candice
Mostly because she is an Ice Type trainer, but she also had a bit of a bigger part in the anime with her being friends with Zoey and the others.
Unova - Skyla
Like I said, overall she is probably my favorite. I love her style and name and she uses flying types. I do wish she had better pokemon, but hey, she is still great.
Kalos - All of them.
Kalos is the first region where I've liked every gym leader for one reason or another, but most of the like for them comes from the anime. Korrina has been a major rival for Ash. Ramos had the celebration that brought Ash and Serena together. And then Wulfric made Ash get mad and had another scene with Serena which I love.
Alola - Mallow.
Love her design and I love that she likes to cook, which is one of my favorite things to do as well.
Galar - Bea
Once again this is mostly because of the anime and her love of fighting types. She was the first challenging female rival for Ash.
Paldea - Iono
She is honestly the only one I remember the most about. All the others were very forgettable. Larry I can remember, but I just don't like how boring he is. And I've been surprised how much he is liked.