The Sideshow exclusive AvP Ancient might actually be rarer than the Asia exclusive, since you basically could only get them directly from SSC. The Asia "exclusive" seems to have been available everywhere. I'm not sure why they called it that in the first place. Scar and Celtic went pretty fast, being the first two releases. By the time Chopper and Elder came out, those two were basically gone already.
Just my guess, but Battle Damaged Scar may be the rarest of all, at least it seems to fetch the highest prices. Does anyone happen to know what the exact edition size was for that figure? Was it 200 or 500? I seem to recall the Ancient Asia edition was 500 total, but not sure about that.