Kind of depends. Dimension Hatross[/ii] and [i]Nothingface are typically regarded as their best. They both blend thrashiness with kind of the spacey prog feel they got known for, but I never really could get into them. I like War and Piain best, but it's their most thrashy album with pretty well no prog elements (mind you I haven't listened to Voivod in over a year so I might like hte other two more now).
After that, I can't help you much. General opinion tends to be that their self titled is pretty good, but I've never heard it. After that, opinions vary. I remember one big Voivod fan telling me that Killing Technology and Angel Rat were among their best, but I think he's the first person I've ever seen voice that sentiment. ROOOOOAAAAAAARRRRRR is supposed to be good too, but again, I've never heard it. For a good introduction, it's probably best to go with Nothingface and Dimenson Hatross.