I've bought, sold and traded hundreds of guns in nearly 30 years of knocking around this game. Only 3 handguns have I ever regretted getting rid of.
A custom (done by Larry Kelly at Magna-Port) Smith model 29, 3" round butt, Magna-ported barrel, custom grips and a hard chrome finish. It was my EDC gun for a couple of years when I worked at a gun store.
A Smith model 27, 8 3/8" barrel, pinned and recessed. Probably the most accurate handgun I've ever fired.
And the worst, a Colt 1911 (no A1), WW1 era that I got from the widow of the Marine Officer that carried it through 2 conflicts. I even had his holster. I shot the old war horse some, and then a collector liked it a LOT more than I did and made me an offer I couldn't refuse. I sold it for enough of a profit to buy 4 VERY nice guns. I REALLY wish I had that one back.
I've also regretted the loss of a few long guns (mostly a 375 H&H), but these are the 3 handguns that I most regret letting lose of.