Unova Gym Leaders are my favorite group of Gym Leaders as I don't think there's one who didn't appear outside of their Gym or had some small part in the story (at least in Black & White).
As for the music, not only did each Gym have its own theme but the themes were creative remixes of the Gym's theme with each giving the Gym its own feel.
Aspertia Gym: The original theme from Black & White. Would make sense for not only the the Normal-type Gym to use the plain theme used in the previous game but since its the first Gym it provides to familiarity going from BW to BW2. Also being Cheren is a new Gym Leader he probably hasn't really got into his own groove yet, you can even see the Gym isn't completely built yet so there's no theme to it yet.
Virbank Gym: OH MY ARCEUS! VOICE ACTING! Yes, one of the themes that probably stands out the most, at least at first. Not only is it a rocking tune to match Roxie rocker attitude but the voice "singing" also sort of helps elevate BW2 from other Pokemon games in general. I joked about it but the presence of actual voices is a first in Pokemon I think, in addition the voice is suppose to be the Gym Leader's (plus her back-up band). Another neat touch about this music is that it's dynamic, it loses layers to it as you defeat the band members until its just Roxie singing and playing on her bass. Makes it feel they're actually playing the music instead of it being something the player hears but not the in-game characters.
Castelia Gym: This theme actually sends shivers down my spine, but in a good way. Its creepy and ambient tone really fits with the Bug-type, it makes it feel you're in an alien structure. Burgh also was always a bit of an odd guy, I mean its kind of add that an artist would specialize in Bug-types, but I think this theme builds upon his joyful excessive personality; like a child doing something they think is cute/cool but is a bit creepy to everyone else.
Nimbasa Gym: Virbank Gym 2.0. Has Voice acting and dynamic music that builds up to a grand entrance by Elesa. It perfectly fits Elesa's Gym being a fashion show as well as her extraordinaire and grandiose personality (aka big ego, aka Lady Gaga).
Driftveil Gym: A beat heavy theme which gives the feeling of you're going through some kind of production facility like a factory or, say, a mine. The beats sound like workers doing their daily grind and machines whirring noises. And it could just be what I'm associating with it, but there's also something about it that feels it belongs as an underground theme or someplace that's dim. Goes well for Clay as he's one of the more serious Unova Gym Leaders.
Mistralton Gym: An airy theme, makes sense for the Flying Gym. The theme more applies for Skyla's flighty personality than the Gym itself, however the background of wind noises can be related back to the Gym's theme of blowing you away with high powered turbines.
Opelucid Gym: An epic theme fitting for dragons. Goes well as you rise up the dragon's body, fighting through trainers and their dragons along the way, until finally getting to the head where the Gym Leader and the Gym's most powerful dragons await. Drayden is a very imposing man, this theme giving him a bit more feeling of intimidation.
Humilau Gym: Another gym more reflecting the Gym Leader's personality. This is a theme you just want to laid back to and let time fly by while you just doing nothing, very much Marlon's attitude. Not to say it doesn't go with the Gym, the water and beach can be very relaxing especially when you're riding on giant lilypads letting them take you wherever they were pushed to. There's no need to rush (well aside from Team Plasma firing ice cannons at cities), just solve this Gym puzzle at your own pace.
