Forgettable: Hoenn gym leaders that aren't Tate and Liza. Partly because I recall them being the only double gym, and partly because they were mentioned in the OP. The rest though... I have no idea who they were.
Memorable: I'm ignoring Brock, Misty, Cilan and Iris here, because it's difficult to forget major cast members from the anime. I'd say Roxie or Elesa are the most memorable.
I don't think it's because of the anime. Brock is memorable because he's the first gym-leader people who started playing R/B/Y/LG/FR faced. If you started with a Charmander it was even more memorable just because of how much grinding was needed to beat him. The same goes for Misty even though she's the second leader.
Afterall, you don't see Clian or w/e the green guy from Black/White's first gym is being listed here and he's traveling with Ash now.
Actually, that is why I ignored him. I like Cilan and he would've been #1 (especially because of his "dance" in B/W1) but I went for Morty instead.
Official Spike of the PSASBR Board Fan of a "pretty hardcore evil" Vampire Queen.