Two things I really regret letting go of....
My very first kit RC, a Tamiya Lunch Box. Fragged the tranny with a Trinity Nuclear Assault motor, and never repaired it. Traded it for a Tamiya Striker (dumb, dumb move)
The second is my RC10 w/ Hauge chassis and trailing arm suspension.
I had upgraded it to the stealth tranny (when that was the new thing, ha!) and kyosho platinum shocks.
Only wish I had pics..