My regret is selling my Intense Slopestyle mk1, particularly as I sold it only 3 years ago and for not much money. Great park bike and for hammering round the Surrey Hills here in the UK. A tad short, but fast, burly, poppy, and in raw aluminium looked very “factory”.
Going back a lot (lot) further I sold a bike with PowerGrips. These were an alternative to toe clips and pre-dated SPDs as far as I remember. I’ve ridden flats for the last 10 years and wouldn’t go back, but would be great to try them out again! Turns out they still make them!