My scoped Marlin 336w 30-30. I've considered trading every other firearm I own at some point or another except that one. The thought didn't even cross my mind until I saw this thread. I can't think of anything I'd be willing to trade that toward. Maybe a Lightsaber from Star Wars.
A few old heirlooms -
20 ga side by side that's over a 125 yrs old that's been passed down from a great great great uncle.
An actual confirmed german made Walther that was found in my grandfathers belongings after he passed, non matching numbers, all factories have been identified. Story unknown as he never spoke of the war.
His 1911 that was found with the PPK.
Last but not least another PPK/S passed down to me from my other grandfather it was his EDC.
As for the rest they all have their price. Sorry it was a list and not just one!
1934 Beretta. It was given to me by my grandfather. He bought it while he was stationed in italy during WWII. The bluing has almost all turned to patina. It's not worth a whole lot but the sentimental value is priceless.
Everything except my P22 presently. I overpaid for my Sig P229R and it was my first gun so I'm nostalgic about it. I've got a bunch of money at a discount in my AR as well as a few mods on my EDC M&P40c. I know I'm not getting my money back out of those and like them too much anyway. I actually traded a P22 for the newer extended P22 I have now and still don't like it. That's about it as far as anything I'd get rid of, but I'd like to adopt the "never sell a gun" mindset anyway.
My dad's uncle Dewey gave a single shot shotgun of unknown manufacture to my grandpa on the occasion of his marriage to my grandma in 1922. The pattern of that twelve guage is such that I haven't shot it in years for fear of screwing something up. I'll probably run a couple of.shells through it sometime in the next few weeks to show my kids what its capable of. Wow. It'll never ever be sold. I have a few other won't sell heirlooms, but that one's the queen.