In general, I'm a sucker for songs that have nice melody. I listen to mostly J-Pop and J-Rock... Cause of anime.
The new spring anime, Claymore, has a really nice ED song, Danzai no Hana, and the singer Kosaka Riyu also sings Ignore that's in the same album great songs. ^_^
I'm a Haruhi fanboy, so I really like all of Aya Hirano's songs. God Knows is awesome, while Lost My Music has its own charm.
I became a fan of Janne Da Arc cause of all of the Melty Blood combo vids on youtube.
Vampire, Tsumetai Kagerou, Wild Fang, Red Zone, and Ring are some of my favorite songs. Vampire has nice melody, and when Tsumetai Kagerou is playing, it feels like you own the place. 8)
I like Two Mix, starting with their OP's for Gundam Wing. Being, the second OP of Shakugan no Shana, made me a fan of Kotoko.
Nami Tamaki I really like, beginning with her OP for Gundam Seed, Believe. Reason is one of my favorite songs by her, as well as DreamerS.
Other than Japanese songs, I like techno, euro beat, rock, and stuff. Pretty much I just can't handle rap. =/ It feels like there's no melody line. ;.;