The best voice is AT&T Labs' Natural Voices : Crystal 16Khz version 1.4. It's far better than microsoft Sam. If you prefer a male voice (female is better most of the time, trust me) use AT&T Lab's Mike.
However, in January, when Windows Vista is released, there will be a new engine, SAPI v. 6.0.
It's much more advanced than the currently commercially available voices. Microsoft Ana is the new voice, and she sounds' awesome in freespace. There isn't a male voice in the current beta builds of Vista, but there probably will be one by release.