Wheel Horse - particularly anything from the days that AMC owned them (mid 70's thru AMC's downfall in the mid 80's) Although the Pond (original owners) Wheel Horses are older and more rare, they weren't as good a mower, and don't draw the interest or the money.
Also Ford/Jacobsens, 70's Simplicity/Allis Chalmers, 70s Massey-Furgeson, and Simplex garden tractors.
They seem to get TONS of money, and I see more and more of these very machines at tractor shows.
As far as a modern machine, I'd bet the Weed Eater One is going to be a classic someday. It's a terrible mower, but they are selling them by the thousands. It's going to be an icon, I do believe. If you have the place to stash it, I'd grab one for posterity. I am not kidding here. There won't be any survivng examples in a decade. Look at the Huffy's and other AMF-built toy mowers. They were terrible, but they are so rare, people gobble them up.