Most expensive:
HK P7M10. I'd owned several P7s before and this was a "grail gun". Finally got one (actually, found a consecutive numbered pair), and it was just like any P7...except a bunch bigger and clunkier. I don't think I kept them a year. Good news...made a buck or two on that deal.
Most (potentially) unpopular choice:
Glock 23. Had it for ten years and probably didn't put 1000 rounds through it. Just never got enthusiastic about it. Traded it for a G19, which I kept for a year or two, and then sold.
Biggest disappointment:
Springfield EMP. Partially my own fault as I bought a very early example. It was ridiculously unreliable--spent as much time In transit, and at Geneseo, as it did in my hands. Sold at a serious loss, but I have other 1911s that keep me happy.
Longest run:
Had a half-dozen S&W Scandium revolvers. Loved the concept, bought in (did I ever)...but eventually figured out that they weren't getting any easier to shoot well. Sold them all and bought a few older (pre-ILS) S&W revolvers, some P&Red. I actually enjoy shooting these revolvers.
Probably a few others, but that will do for now.