For me, it would have to be my FEG Hi-Power. Traded my first pistol, a S&W Sigma 40, for it shortly after deciding that polymer frame .40s and I don't get along.
I did all kinds of work on it, at least for a broke college student who had little-to-no gunsmithing/armorer experience. Swapped out the extractor, new springs all around, a little polishing, painted those stubby little sights with glow-in-the-dark paint. I was as one with that gun as any I'd ever had. I shot enough back then (when ammo was even cheap for college students) that I was very accurate with it.
Then, I decided that I had to have a .45 ACP. I'll still admit I needed one - everyone should have one - but I didn't need it enough to sell that Hi Power. But that's what I did. Swapped it for a stainless Springfield 1911, the base model, as there's no way I could have got a TRP or one of their better models. It was an OK gun. I was reasonably accurate with it, especially after swapping to a better trigger. It was reliable, as long as I used factory mags or Wilson Combat mags. I changed out about everything I could on that pistol, without real gunsmithing like fitting an extended beavertail to the frame.
But it was just never the same. I missed that old Hi Power with its kinda tarnished bluing and those old familiar grips. Even started missing the hammer bite.
So, I finished college, moved back to my hometown, and got a real job (and a haircut!). After getting back on my feet, I finally made a trip back to that old gun store, just to catch up with the owner and see what they had. To my surprise, my Hi Power was still sitting on the shelf.
I paid more for it the second time (don't tell the owner I'd have probably paid double!), but it came home with me that day.
I had always wanted a real Browning, and I finally got one a couple years ago. I still prefer my FEG.