I forgot to mention SID, He was amazing in the Magi OP.
ZAQ are pretty amazing too especially when they sang the Chuuni Lite OP
FLOW is wonderful and same with SID. I love Lia for doing those Key songs. The Pillows just plain rock. I really enjoy Kanako Ito's songs from Steins;Gate, and Megumi Hayashibara's songs from Slayers. Shihoko Hirata's songs from Persona 4. Porno Graffiti. More recently, Sukima Switch, I liked their song from FMA Brotherhood and love that opening they did for Space Brothers.
ZAQ did the opeing for Chuunibyou. That's a really enjoyable one to me because of how my strength she puts in the chorus. She really belts out and yells it and I love it. She also did the opening for the new show Sasami-san@Ganbaranai.
I also want to mention Iwasaki Tarou for his work on the Gurren Lagann/Katanagatari soundtracks even though that's a bit off topic.