Hmm, let's see. I gotta pick 5 artists first
Gackt - Vanilla
I wanted to try out some male Japanese singers, and had heard lots about gackt so i checked online. First song I found was Vanilla, and I HATED it completely. I deleted it and waited another 6 months before listening to him again

. I still don't think it's that good a song even now.
BoA - Number One
Lol same as Sven's comment
X-Japan - Sadistic Desire
Like with Vanilla, i HATED X-Japan's Sadistic Desire, i thought Toshi couldn't sing and just disliked the whole package. Fastforward a few months and I'd become a huge fan of the band and i actually love that song now
Bonnie Pink - It's Gonna Rain
I heard this on the end of Rurouni Kenshin (i think it was the second or third ending song). I loved Bonnies Voice and still do today, she's one of my favourite singers ^^
Noriko Sakai - Pure
Nori-P, the FIRST EVER Jpop singer i listened to, which would in effect make this the first Jpop song i ever listened to. Such an excellent song from such an excellent singer. Still found floating around on cd's scattered throughout my house ^^