My 2 cents. It really depends on the role of whoever I'm playing. As a healer, I like obvious (but not obnoxious) group bars, preferably with a good contrast between the "gone" hp and what's left. When I'm soloing, the only real info I need is my hp/mana, pet's hp and a few commands, and the enemy hp bar.
I prefer a clear window though...being a cleric/healer is boring, if I can stare at something not related to everyone's hp, I probably will.
Also prefer one that I can stuff up at the top of the screen, as my chair sits high/my desk is low, to make it easier to keep up with what's going on. (And now I'm just so used to it.) Been using the Vertibrae skins for some time, it has everything I need and doesn't eat up too much screen space.