I'll just say my opinion on each of them, because I'm too scatterbrained to pick a definitive favorite and least favorite! :D
Chili Cilan and Cress - Honestly my favorite part of them were the bad puns and wordplay. I do love me some bad puns. As characters their design and battle style doesn't interest me much though.
That Normal type Librarian Lady - I appreciated her role and existence in the game. It seems minute too; but I'm just glad they finally put a black gym leader into the mix as well. (NO Brock is... some kind of, Idunno something besides black though. Maybe... The same thing as The Rock. HAH ironic.)
Burgh - he uh, well I liked his pokemon and battle more than him as a character. In the show it made me wanna swallow my fist when his Leavanny knew Hyper Beam. :I Why Burgh... why....
Elesa - Not really one I liked at first, in fact I thought little of her in B/W 1. BUT - in B2/W2, through the memory link - I got to see the most amazing cutscene in pokemon ever. The discussion between Elesa and Skyla. I learned that Elesa secretly did not like her life as a model, and yearned to be taken less seriously - and make TERRIBLE jokes and puns. This tickled me to no end, and instantly made her my favorite gym leader. "Throw that misbehaving Klink in the Klink!" DOHOHOHOHO
Clay - Meh, Clay was kinda cool I thought - because he was one of the first to run in and fight the Seven Sages at the end of B1 / W1, which seemed kinda awesome. Plus, his pokemon were neat. Excadrill besides being banned, is cool.
Skyla - looks like Sylveon / Ninfeon. :I Also, appreciated her design - but as soon as I saw it, expected the more adult pokemon community to um... Well, indulge themselves in a lot of fanart with her character.
Brycen - I liked Brycen. He was given an awkward gym leader role, being the Ice type leader, where even his little welcoming guy kinda told you Ice types had a lot of weaknesses (then proceeded to list them all off... that guy should be fired.) But he also had a signature pokemon, that was physical - and ice. His Beartic was pretty intimidating though. Still, Gigalith lol'd at this.
Drayden - I disliked. :/ I seriously spent about, maybe 2 hours looking at his sprite, character art, and fan art of his face... because I couldn't figure out what the buck was going on with it. Is that... a beard? A chin strap thing? Is he trying to be Bane from Batman? Did he like, bury his face in whip cream and decide to just go with it, and leave it on? It wasn't until my friend in the opposite version - Spitfire - told me it was actually supposed to be a beard, I gave up on liking him. That's not how you wear a beard, Drayden. :L It looks like that thing is trying to eat his face, and the two are engaged in a neverending battle. That he's losing. Seriously, I wanna hear him talk. It must sound like "Mmmff pmmf mahhhfmmfm."
Cheren - I'm glad Cheren was not only back so soon in the sequels, but also played such a significant role. I wasn't used to that much character development and importance in a Pokemon game, and it surprised me. Plus I like his pokemon, and going around with him when you team up.
Roxie - Roxie was legit. :D I liked her, her design, pokemon, persona, and battle style. She's coo.
Marlon - when i first saw his character pre-release... I shuddered a bit not gonna lie. Then I played the game and found out he talked kinda like I did, and was basically the biggest bro in the entire region- and what's more, a pretty solid gym battle. Respect, blue haired half-wetsuit wearing guy. Respect.