The one game I regret selling is White Water. Great game in really nice condition, and they've become much more expensive / harder to find since then.
Of the other machines I've sold I'd also quite like another AC/DC Premium at some point, but I don't regret selling it - I really enjoyed it but at the time it wasn't really getting played much and I was ready to move it on. Now that it's been a while I think I'd enjoy owning one again, but no regrets.