my absolute favorites are
L'Arc~en~Ciel above all the others
try: LOVE FLIES, Hoshizora, STAY AWAY (I recommend this PV as well
), Lover Boy.
with these songs you should have a nice foretaste as to what to expect from L'Arc~
and if you're into these twisted 9th and 7th chords like me than you're going to love Ken's guitar parts
Dir en grey is awesome as well, but needs some getting used to for some ppl.^^'
I would begin with their older stuff and then continue with their more recent work
for female singers I would definitely recommend
Shiina Ringo
try: tsumi to batsu, gibbs, Akane Sasu Kiro Terasaredo..., yamini furu ame, yattsuke shigoto
she's got also quite some cool PVs