This is going to be a little offbeat looking at the rest of the posts... but one of my favorites (I have a favorite in each sub-genre) is Ua.
Ua is a very independent lady and has a jazzy blues style I like very much. She's also quite the experimentalist ranging from cabaret to avant-garde unlike most singer/groups that find a rut and stay in it.
For pure fluff I find euro-jpop cute... Hinoi Team and Zone are fun to do house and yard chores to (and they're pretty damn cute --- cute wins
).
I also like some of the voice actress Asakawa Yuu's work ... unless most VAs - she doesn't follow the rut in that poppy sappy route: some pretty hard rock and jazzy stuff. I guess she's the "bad girl" of VAs
and she's attractive in a cool way.
I'm having a real tough time finding what I'll call "hard rock" japanese style. I've got one cut from a character CD that almost fits the need (the track from Negima's Evangeline CD). Something along the lines of Zepp, Tull, AC/DC, ZZTop ... something with a little power to it... not just industrial or metal. I've heard a bit of the long ago group Shonen Knife ... they're almost there.
I've been told to check out L'Arc~en~Ciel so I'm looking for some samples before I buy.
In many cases, I find the singers have great voices but they're given crap to sing. The singer (Lia) that cut the acapella version of Azora in the AIR OST has a soaring voice but all of her musical release work makes me ill (forgettable poppy sugar).
Ah well... re-listenable music is a never-ending search.