Hatsune Miku, Otomachi Una, and Bert Gotrax.

I tend to have stronger opinions about producers than singers though, and since vocaloid "singers" are just software with an associsted character, the producer is what you want to look for. On that front, look up Astrophysics, Kikuo, and Patricia Taxxon (none of these are exclusively vocaloid producers, but their songs that use vocaloid are very good.)