The obvious loader--but made so much better through chain hooks & 2" receiver.
Box blade--go to compact counterweight--also better with chain hooks and 2" receiver (but not as much).
Bush Hog/rotary cutter, despite issues with the brand & model. I regret that cutter, but couldn't do without one. The grass waits for no man, so for 6 months, it is always on-deck if not on back.
Pallet forks--for moving large heavy stuff without the up & down on & off the tractor; for moving stuff that can't shift I added hooks. (also better with hooks--I may be hooked on hooks...)
Backhoe. 90% regret--so much $$$. A good counterweight, but not a great COG on slopes. If you've ever dug a trench by hand, and never used a BH, then had a chance to use a BH--I definitely smiled--may have laughed or cried. No substitute when needed. I just realized--it probably needs a hook or two...
Logging winch. 95% I regret, because it was a lot of $$$, but it has been indispensable for rough tasks: it's taken down widow-makers I bungled the cut on or discovered looming above areas we pass. For harvesting wood my life is so much better than chaining, trying to balance the load and skidding 2-4 logs off the box blade, or attempting to carry sections on the FEL/forks/bucket inches from the powerplant--constantly dealing with elevated weight shifting as terrain changes is terrible. For getting unstuck, or getting someone else unstuck without entering the myre, 150' of cable is nice. Levering out small root balls and stumps that would eat up time and an implement change... watching 4 logs or uprooted root balls shuffle toward you brings a smile.
Wood
chipper--98% regret--awkward PITA to put and have on--essentially makes the unit useless for any tasks outside chipping... irreplaceable for getting rid of smaller slash and putting wood chips in my landscape. Also tends to make me smile as it chews through something that probably should have gone to the wood stove.