For me it was the MMM. I got a great deal on Ebay on the mower but never used it, couldn't get the MidPTO working. Even if I could have got it working I'm changing/removing the FEL, Backhoe, rotary mower so often, the MMM would have been a pain in the butt, plus the mower is the same size 60", as the one on my Garden tractor.
This thread makes we want to start one on the best and worst tools you ever bought.
Curt