>For me it would be my T160
Me also, but no "back", I've never sold it.
The only bike I've ever sold and had any regrets was a Honda 250RS but, at the time, it'd buggered its big-end. In theory, it shouldn't have been too hard to fix, given its built-up crank and roller big-end are basically-similar to many Britbikes. However, Mate O'Mine who rebuilt it said Honda included a few things to make rebuilding extra-difficult ... nevertheless, he persevered and did fix it.