I voted for HYDE.
Gackt is doubtless talented, but he's too robotic. There's never a spontaneous moment with him; everything seems calculated or contrived. He's also blatantly narcissistic, which is rather obnoxious and turn off-y.
Voice-wise, Gackt is good. It's not anything that'll pull at my heartstrings, make me shed a tear or whatever. His incessant vibrato is rather annoying, too; he needs to learn how to control it. If it's deliberate, then...I really can't say anything, except:
WHY?
(One word: Vanilla. *shudders*)
I do give Gackt props for his music, though. It's pretty good. A talented musician, I can't dispute.
hyde appeals to me more because he comes across as the genuine article: what you see is essentially what you get, but with that underlying aura of mystery that makes him so attractive.
Voice-wise, I share the same sentiment as everyone else: there's more raw talent. It's wild, un-harnessed...beautiful. If you listen to songs like "hitomi no jounin", "Anemone", "As if in a dream", "finale", "ibara no namida", "hoshizora", "yokan", anything off ROENTGEN, you'll understand where I'm coming from. Overall, the proper adjective to describe his voice would be "haunting". And yes, I, Mari, The Ice Queen, have shed a tear or two listening to him sing.
Also, some of my favorite L'Arc songs ("ibara no namida", "LORELEY", "Sell My Soul", etc.) are ones hyde composed, so he definitely commands my respect in regards to his musicianship.
And now, ladies and gentlemen, the final judgment: LOOKS.
HYDE > Gackt.
back in the day...
Ser'ously.