Top 5 for me would definitely be:
UVERworld
nano.Ripe
Do As Infinity
supercell
LiSA (Risa Oribe)
All of them have done songs for at least one anime, and some songs I highly recommend any J-pop/J-rock fan to listen to are Hakuchumu and Kimi no Suki na Uta by UVERworld, Hana no Iro and Tsukikage to Buranko by nano.Ripe, Rakuen and Yotaka no Yume by Do As Infinity, Kimi no Shiranai Monogatari and Hero by supercell and Shine Days and My Soul, Your Beats! by LiSA who sang the songs for the 2010 anime Angel Beats. Just a couple of songs that are at the top of my playlist for the J-pop/J-rock genre, cheers!