A very interesting question with interesting replies.
To answer the original posters question, for me it was the forks for the FEL.
I looked high and low to find a used set of forks so I could build my own set. I've had them (and plans to build) for almost 2 years now and haven't needed them yet. I do have a forklift truck in my barn so maybe that's why. But I envisioned using them out in the field on the tractor where the forklift truck would not go ... it's not an off road model.
But there again that's only for me, in my case.
I guess it really depends what you mostly use your tractor for and what is the least you use it for. That will (to some larger degree), dictate what implements you use most. And it will vary from year to year as your place comes around from what it was to what it is, and what it's going to be in the future. Following suit, it also depends on what your place was like when you got the tractor and implements you thought you needed.
In my case, my property was a wild Oak forest on a sloping piece of property.
I pretty much needed one of everything to do what I wanted to do. My needs lean towards building roads, clearing, pulling trees, ditching, footings, building pads, preparing to have my home and barn built. I did not need any farming implements (yet) since that was on the back burner. When I did need to plow I improvised and used the boxblade for that too. Now we have a tiller because things have changed (again).
I'll bet my box blade (for me) along with the FEL and bucket grapple have been the most used.
Then the backhoe. At the time, the added bucket grapple and backhoe were life savers for me. My sloping ground need an implement that I could "tilt" different than the wheel base to build roads and trails on my sloping property. That way I could contour and not just follow the lay of the land. Like banking my roads in the turns cutting into the hillsides. But again, that is because my set of circumstances required those types of implements more than others.
No doubt your needs may vary, of course.
Rob-