Hi,
I'm not sure what you mean by "library of MIDI voices". Are you looking for sounds for a specific synthesizer that you are using?
If pads and atmospheres is what you are after something like ILIO's "Atmosphere" is probably what some call "Best of the best". Mind you there is no such thing. IT's more like, "okay, I like it so it's good" or the other.
IN other words it is very individual. There are no good or bad sound libraries out there. It's all in what you're after.

-ED-