As far as singers go, for me it would probably go:
#1: Rita
#2: Rekka Katakiri
#3: KOTOKO
Tenshi no Tamago’s OP first got me interested in Rita in a big way, and the more Rita songs I listened to the more I liked them. Tamayura’s OP got me into Rekka Katakiri, love her work-- she does a good job with the sort of eroge OPs Narg likes =p (high energy works, whereas Rita’s work is a bit more ‘peaceful’, for lack of a better term) KOTOKO I can’t even remember, but she was the first eroge OP singer I started to really follow. Probably had something to do with the Baldr Force, Natsuiro no Sunadokei, Duel Savior, Triangle Heart 3 and Hajirahi OPs. The only KOTOKO album I ever got was Hane though. (Hane, the song, is great and I still love listening to it every now and then)