The only games I regret selling are the original NES copies of Dragon Warrior I, II, III and IV.
Not because they represent some pinnacle of gaming (although they are all pretty cool) or because I can never play them again (I have them all for handhelds-I, II and III play fine on my Gameboy Player and IV plays on my 3DS.)
I think it has more to do with what those particular games, as well as the original NES Final Fantasy (which I still have complete in very pristine shape), represented to me at the time they were released.
I was playing a lot of RPGs on the C-64 at the time. A lot of Ultima and early EA offerings, like Standing Stones, Bard's Tale and Wasteland. Those games represent a very magical time in my life, when I could lose myself for hours upon hours and not have to worry about things like work and bills. My very own Never-never land, so to speak.
But computer RPGs began evolving to the point where the C-64 couldn't run them anymore. Not the ones I wanted to play. Ultima VI ran like ass on my Commodore and my disk drive started flagging at the same time. My only option was to buy a PC, but I didn't have the cash for that.
Then along comes the NES. Granted, the RPGs weren't nearly as robust as what was available on the C-64 (at that time) but their simplicity and the ease of play with them really struck a cord within me. After using graph paper to make so many maps, hand writing spell ingredients and recording whole journals of game notes just to beat those games, the NES provided a simpler alternative.
Add to that that I was really falling in love with anime and manga at that point in my life, and many of the support materials for these games (manuals, strategy guides, Nintendo Power articles, etc.) had that exact art style and my mind made up the difference in what the games looked and sounded like.
So from that standpoint, and what the DW games were at that time, I really wish I still had them. I still love breaking out my old FF cart or my gold Zelda carts and just thumbing through the included materials and inserts. It's a tactile experience that takes me back in a way that a 'greatest hits' version or a compilation just can't touch./
But other than those DW games, I can't think of a single game I regretted selling or giving away. So obviously, it has to be something really special for me to care.