I don't know where I originally got the numbers but I have them saved to my PC. I have the shipped numbers, not the sell through ones (some DC games sold less then 1000 copies in the US before being clearanced out. Once games reach that phase of retail, they often don't continue counting). I don't have the numbers of Midway Arcade 2 though... it was a pretty late release. I'm guessing it's only uncommon as before the fall of the Dreamcast I saw tons of copies of it everywhere. However, there is only two active auctions for it right now.
Soul Caliber was the greatest selling DC game, and in fact the only US DC game to sell 1,000,000+ copies. Aroudn 1.25 Million were shipped into the US.
Marvel/Capcom 2 is
not rare. 100,000+ copies were shipped, and the value has now dipped under $30. It will continue to devalue, like all other 'false alarm' games.
Bangai O had around 5,000 shipped and it's large volume on Ebay proves it's not very hard to find. A true rarity would rarely, pun intended, show up on Ebay.
Diddo for Industrial Spy. Although it's the lowest produced DC game (2,500 copies or less), it's
neither hard to find or valuable on Ebay or in the wild.
On the other hand, Aerowings 2 seldom appears on ebay. There are two active ones going right now (and NO completed ones). With about 5,000 shipped, it's hard to comeby. Snatch it up if you want a good Dreamcast collection (good as in good hard to find, because Aerowings 2 sucks).
ECW Anarchy Rulz shipped about 5,000 at a time when Acclaim was hurting. Once again, you see it all the time, and it's not worth any money.
Here's a funny one: Kao the Kangeroo. Shipped a very quiet 5,000 into the states, and it ended up selling four times that (20,000) in Europe. Many people bought this from Gamestop or EB for $10. It's pretty hard to come by.
One that nobody has mentioned yet is BANG! Gunship Elite. I don't have the shipping info on this one but i will say that only 2 have been on Ebay in the last couple months, and they're going right now. Cheap too.