My 08 vintage Gen3 Glock 19 is the only gun I would never sell under any circumstances, unless I couldn't afford to eat or something insane like that. It would be in the cardboard box with me in the tent city, LOL. Even if someone offered me $2000 for that gun right now I'd probably tell them politely to go screw. And I know that sounds insane because it's just a $500 pistol... but its one that I would quickly trust my life to no doubt about it. It has never f***ed up, not even once. I put an OEM 3.5 connector in it, set of trij night sights (mostly because I wanted sights that weren't plastic, not for "seeing in the dark" I view trit lamps as a "find my pistol in the dark" thing more than a seeing the target thing... ) Glock ext slide lock, Vickers mag release... It runs everything from shit weak ammo to the hottest +P+ that underwood sells. I think I replaced the recoil spring in it once. Probably has like 4K down the pipe.
As far as the original question goes though, I have to say "pretty much everything I own"... but if I had to pick just
one... it would be that one.
-Mike