There are 3 I miss most having sold on…
Schwinn straight 8, awesome DH bike, sold it for next to nothing when kids came along… Should have mothballed it and got it out again years later!
Kona howler. Not quite surenwhy I sold it, and would be brilliant now that 1x drivetrains and big cassettes are a thing.
Original dialled love/hate. Built as a light singlespeed, with magura 100mm forks and fat light tyres. Sold it to get an on-one ss 29er which was more capable, but nowhere near as much fun.
4th wasn’t an MTB, but my Kona big Mama race BMX. Sold it as a dodgy back stopped me doing much BMX, but it would be nice to cruise around on with the kids.