Top voices for text to speech I have heard so far by branding.
Acapela Voices http://www.acapela-group.com/text-to-speech-interactive-demo.html
ATT natural http://www2.research.att.com/~ttsweb/tts/demo.php
Nuance RealSpeak Voices http://www.nuance.com/vocalizer5/flash/index.html
L&H & True voice , being at the bottom, unless they improved it lately.
(not sure where to place it) Cereproc http://www.cereproc.com/support/live_demo
I find that the "UK" type english voices can sound "better" from my american perspective, than the "american" voices. Either the accent is hiding more of the problems, or i do not know enough about UK inflections and nuances to be as critical of it.
All of thse will run on the windows system, using SAPI 4 & 5 .
Voices are not all that is needed, for perfect voice though, a good program with dictionaries, pronunciation editing, and the usual tuning of the voice for specific words is going to be nesssiary if you want to get closer to sounding like a real human.
This location http://www.nextup.com/TextAloud/SpeechEngine/voices.html has a lot of samples you can hear of voices. It was a good compilation of the different voices.
The best of the best voices they say (I have not heard them yet) do not work with the system alone, they work only through the seperate program for the voice. The program and the voice are needed and work together. I have not found it yet.