Oddly, I found the voice acting for the TEC and the Advent to be top notch compared to other games (particularly the voice acting and script in Starcraft II).
To me, the Advent voices represent a chilling combination of enlightened child and serene, iron-willed adult; they also get bonus points because they're not at all coy or hypersexualized, like women in other video games so often are.
Likewise: to me, the TEC voice actors for the Sova, the Akkan, the Kol, the Dunov, the Kodiak, the Protev, and (post-Diplomacy) the Cobalt all conveyed a sense of calm, rank-and-file citizen soldiers who would be right at home as competent officers in any space opera (whereas the Cielo's voice is just AWESOME -- "I've. Been. Thinking."). I have some issues with the Marza, the Percheron, the Garda, and the Ogrov: to put it mildly, they seem to be chewing the scenery a bit.
I'm ambivalent about the Vasari. To be honest, the Protoss voices from the original Starcraft, richly laden with harmonics throughout the audio spectrum, strike me as having some of the highest production values in video game history in the field of alien voices, and I rather wish the Vasari followed their example (as opposed to sounding like they've been gargling bile).
Likewise, the Beast voice from Homeworld Cataclysm is an excellent example of an alien voice with high production values, given its odd but consistent syntax ("Kuun-Lan parts! We want beacon brain! Give us data, or we take data with your parts!"), the way it seamlessly alternated between a male and a female voice, and the manner in which it was overlayed with beastial growls around the consonants and vowels. Superb.