Without a doubt, White Water. I didn't want to sell it, and I sold it cheap, but I was moving and needed the cash to pay for the moving truck.
For arcade games, only one I regret selling. The Sega Sonar that passed through here last year. If I had known how cool and rare it was at the time I would have kept it for myself, but I had already promised to pick it up for a fellow pinsider from a local pawn shop. Not upset in the least though since it went to a good home and is probably already looking a LOT better than I could have ever done with it.
-Hans