Depends on who you want. @Halo-Halo 's voice is angelic, but he's a dude. If you want a female voice ask @Troisnyx :)
Comments
I'm flattered you mentioned me, Troisnyx :) you summed it up perfectly and I agree that I don't think there is a single best singer. What are you looking for a singer for?
Troisnyx
Best? Depends on what you want.
@hania has done pop, funk and many things besides, and so has @headphoamz, but the two of them have plenty of their own projects and deadlines to fulfil.
@MetalRenard, @bassfiddlejones and @Kor-Rune do rock and metal stuff. @Lich does it to a degree, but applies it to his industrial music. There's also @BlackLikeRain, who is new-ish to Newgrounds but sings quite decently.
If you want ethereal and angelic, there is always @etherealwinds. There's also myself; I do sing.
If you want chopped-up things that go well with an electronic vibe, there is always @kazaki03 and @DreamForecast.
If you want an Oasis-like vibe, there is always @McGorilla42.
And if you want an operatic voice, there is always @felimimimi.
I think, from the musicians' point of view, there's no real 'best singer', 'best this', 'best that', unless it's absolutely clear someone is virtuosic in his stuff.