Obvious picks, but Mikuna Shimokawa and Minami Kuribayashi, but then they must be obvious for a reason right?
Of course Maaya Sakamoto is probably the most popular, and I think she has a lovely voice, though I find myself quickly tiring of her songs, whereas I could listen to the FMP OP/ED singles every day, all day, and I've been known to do so.
I can't figure out why Chiba Saeko slipped my mind... Oh well at least I cought myself right away.