S&W Model 66-4 2.5" ... I was thinking, yeah, I'll always be able to pick one up later on the used market (sure, with bank-vault lock-up and tuned trigger) ...
An inherited pre-Model 10 that probably had never been fired (faint turn line and impeccable bluing) before I put just a few cylinders through it ... If I knew then what I know now ...
Like the OP, a Marlin 1894 in .357 ... boy, was I stupid ...
An original (pre-Series 80) LW Commander from the actual '70s ... again, boy, was I stupid.
A Taurus PT-92 AFS-D, my first 9mm ever that I personally bought (thus totally a sentimental pick), and it was a gem ...
A factory stainless Colt's CCO (Officer's slide, full-size grip) ... it ran 100% for a short-barreled 1911, great carry pistol and was accurate as hell ...
A Star Firestar (.40 S&W) and a Star PD (.45 ACP), both reliable, concealable, accurate and cool to boot ... the Spanish pistols are classics, IMO.
My first issued S&W 5906 with the arched backstrap ... heavy as hell, but accurate and reliable and also just a cool pistola ...
For those that only mention one, or two, or three guns ... I guess I took a lot longer to figure stuff out than y'all did ...