that depends...
chuck,
you have to tell us what you want to do with your tractor, there are thousands of attachments, each will do a specific job well, so you need to assess your property, which tasks that you need to accomplish the most, which tasks you dont need to do at all.
for example i do two major tasks with my tractor, i cut grass and i move material, so i have a finish mower and a rotary cutter, i have a front end loader and a rear blade for moving material (snow, gravel, bark mulch etc...) these two tasks are about 90 percent of what i do with the tractor. my wife likes to landscape the property so a few years ago i bought a post hole digger with a 12" auger, plants trees, shrubs, rose bushes faster than the devil. that is about 2 percent of what i use the tractor for. i have a 3 pt lift to move things around the property, i have a pto generator if the power goes out. i have a tiller to make garden beds. i have a water pump i made into a sprayer.
of the last four attachments, their use comprise the last 8 percent of what i use the tractor for. could i do without them? sure, would i want to? absolutely not. have i bought my last attachment? are you kidding!!!
given my list i would rate the loader as the most usefull. but i use the mowers the most. in your situation it may be completely different.
let us know what you plan on doing with the tractor, i am sure someone on this site has done it before and would give you valuable insight into the proper attachment and its use.
alex