Well, I had a situation where I had ONE Marine unit left and was facing overwhelming odds--I figured I was gonna die so I tried Surprise--and Surprise!--I took a region! Tried it again the next turn and did it again!
Got into combat on another world and Surprise worked like a charm there too! Finally, though, it lost and showed itself to be just a normal knid of attack. (I have seen somewhere on Atari Moo someone who swore Surprise was the way to go.)
Searching my report I see that my two Massed Assaults just gained regions; once against the other Tachidi and once on the Rocks. I didn't keep good enough notes to see how much I outmanned them or what sort of gravity the planet had, etc., but I suspect that Massed Assault WILL be good if you have overwhelming numbers and don't mind losing forces.
There ARE bugs in the reporting and such during combat, but I think that all the different attacks DO have their strengths and weaknesses depending on your troops and thier experience level, the oppenent and their experience level, the gravity of the world--all knowable--and the unknowable kind of regions you're fighting on.
Also, I believe there is something about Military and/or Government DEAs--you have to take each of those region by region. Which is why you want some on your planets--you limit your loss each turn. I'll have to try and find out what I'm thinking of here if no one else comes to my rescue before then!