I actually like Sabrina. (Especially in White2/Black2 in the PokeStar Studios subplot.)
But anyone, Marlon. For the eight Gym Leader in the game, he's a push over, kind of an Anti-Climax Boss, to quote a Trope. (The fact that he talks like a stereotypical airheaded surfer - because he is one - doesn't help either.)
I like Sabrina too! She's my favourite of all the gym leaders, in fact. Although that's mostly because of the anime, in which she was so much more badass than in the games.
Stereotypical airheaded surfer? Same is true of Brawly, and he's also a pushover. Essentially a Fighting-type version of Marlon, with an outfit that's far more boring. Definitely one of the forgettable ones. Actually, a lot of the gym leaders and E4 of R/S/E were not that interesting to me, notable exceptions being Flannery and Wallace and maybe Drake. Other forgettable ones are:
Blue (Just an ordinary trainer, pretty much, and atypical as a gym leader due to not sticking to a type)
Bugsy (I even have a hard time remembering whether he's a boy or a girl sometimes)
Karen (There was just... nothing much to her, except maybe her Houndoom being difficult. But the Team Rocket admin had a difficult Houndoom too.)
Roark (Seriously? Another first gym leader using Rock-types? Booooring.)
Gardenia (Erika was way more interesting)
Candice (All her Pokemon seem fresh out of the adjacent route, and one isn't even Ice-type)
Volkner (he kind of paled in comparison to the other Electric gym leaders, even Wallace)
Flint (nowadays when I hear his name I think Mother 3 rather than Pokemon)
^^And everyone else in Sinnoh's E4, except Lucian, who is one of the awesomest bookworms ever.
Marshal (the only thing memorable about him is his blue mouth)
Caitlin (yawn... XD Actually, I take that back. She's memorable for being boring, especially in comparison to other Psychic-type users.)
By contrast, the
most memorable for me, by region:
Kanto: Lt. Surge in Yellow (I never played Red or Blue). Using only Raichu? THAT TAKES BALLS.
Johto: Damn Whitney and her damn Miltank and her damn crying fit afterward.
Hoenn: Wallace in Emerald. Sure, go in with Electric and/or Grass Pokemon, and he's easy! WRONG. Steven was boring by comparison.
Sinnoh: Fantina. I really like how she appears in contests and just her personality in general. Plus she uses Mismagius, one of my favourite Pokemon.
Unova: A lot of these gym leaders were memorable... But Burgh's room in B2/W2 is a sight to behold, he says some interesting things, and his Leavanny can be surprisingly tough if I battle him with anything other than a Growlithe. Also Elesa with her infamous switcheroos with Volt Switch, plus she's a friggin' supermodel, how can you ignore her?