I regret buying/don't regret getting rid of a Ruger 345, a CZ 75C and a Para 12-45.
I DEEPLY regret getting rid of my wife's guns after she passed. The kids hocked, then lost, every last one of them. Makes me sick every time I think about it..
I regret getting rid of a early 50's Model 10, .38. (that I traded for the P345), and couple of Para 1911's I wish I still had.
I don't regret the ones I gave to friends.. (Just wish they'd shoot em more..)
I guess my wife's Montefeltro 20 ga is the one I miss the most..
Wish I'd picked up that Stoeger Coach Gun I didn't buy in Destin.. Wish I'd picked up that P7 I had my hands on but got ass cramp when I tried to actually lay the cash in the counter.. I sure wish I'd picked up those 3 Saiga 12's that I had an opportunity to get for $400 each, back in early 09! I still want that VEPR 12, though nobody really respects that one..
Man, I've gone through alot of guns back and forth over the years..

I even wish I'd kept that little Royal Blue/Hard Chrome Kel-Tec P-11, (though it shot about 5' low at 21 feet, no exaggeration!!)
I'm glad I got rid of the LW Kimber, believe it or not.. The feed ramp on it was milled into the frame and I ended up polishing it a little too much/vigorously.. Messed it up, but that'll be the last aluminum frame 1911 I EVER buy..
Still wish I'd bought myself a Sig P220 SA. If I get a chance, I'll get a Ruger 1911, then a Les Baer 1911.
Want a M2 3 gun right this minute, damnit.. And a Piston AR, as I've never actually owned an AR, (just borrowed a few from Uncle Sugar)..
Guess I've reminisced enough, huh..
What a depressing question..