Most I have sold over the past 25 years I don't regret, but I've been having a really hard time finding a BTTF which I would like to re-add to my collection. Only other game that comes to mind was my first, Space Shuttle, doubt I'll ever find an affordable one of those either.
Of course if the question was which game do you regret selling because you didn't get near enough for it, then the list grows exponentially!!!