Iron Man: Most games I think long and hard about selling before I do it. This one I kind of just did it without thinking through. I'm very happy with the game I replaced it with, but I do miss IM. The rules became stale, especially the super imbalanced Do or Die scoring and impossibility of DoD multiball, but I miss the battle. Will probably own another one someday...especially now that there are rumors that the code might be revisited at some point. A few minor tweaks would make this an absolute must rebuy.
Avengers: Sold this one out of frustration after not being able to tweak out the last couple issues 100%. It was about 95%, but that last 5% just ate away at me after years of tweaking and adjusting. I absolutely loved the gameplay, but just couldn't let those last couple minor issues go. Would like to own another one someday.
Whirlwind: Love the game, hated the example I had. It was had a nice playfield, but everything else was pretty nasty. It was literally dripping nicotine and stunk bad. I think I got a little bit of cancer ever time the fan blew. I did my best to clean it up, but it would have never been really nice without replacing...well...everything. I'd like a nice one someday.
Honorable mention to CSI, WoF, and BK.
I was thankfully able to fix my two biggest regrets...ST premium I was insane to sell and have since replaced it with a pro, which I'm completely happy with. SM was sold with the intention of getting a nicer one at some point, and that came in the form of a nearly new vault recently. Now I am to the point where anything I sell would most likely end up on this list down the road!