thats a loaded question but the john deere 140's are very hot collectibles as are the patio tractors.63 model round fender 110's are very desirable as collectibles.deere collectibles seem to be the highest dollar tractors on average but there are other rarer old tractors out there to collect.
Panzers are highly collectible too,as well as being one of the most ruggedly built small tractors.."cheapest" one I have seen for sale was 650 bucks recently on CL..
"Economy" ones too,bring big bucks here..saw TWO of them for sale for 1700 bucks for both on CL,and both of them had the dual transmission option..pretty rare to see two for what ONE sells for around here most of the time..
BTW,I saw a JD 110 on CL's "FREE" colum today !!..located in "Hyde Park" on Boston..not a neighborhood I'd be willing to go too,but I'm sure some lucky Bostonian already has taken posession of it..ad said "it runs but I dont want to mess with it",or something close to that!..
I bet it turns up for sale on CL or E-bay in a few days??..
Pretty much ANY brand from the first year or 2 of introduction, like 1963 Deere model 110 (only 1000 made), or IH Cub Cadet "Original" from 1961. Many machines from back in the late 1940's and early 1950's (post war industial boom) like Bolens, Speedex, Gravely, Panzer, Snapper, etc. Stuff that was intentionally marketed as being "collectible" (Simplicity NFL team tractors as an example, or Bob Vila "Signature" series machines) are generally not a sought after collectible unless you decide you personally have to have one. That kind of collectible is essentially the same as the regular tractors of that year, the previous year, and next year's machines with some special graphics, paint & decals.
For me tractor I want to add to my collection are a 1966 JD 110 with Hydraulic lift, a JD 318, a140H3 Patio in Spruce Blue, A JD 214, a IH cub cadet RED 782 made before April of 1981, and finally a late 1990's Simplicity Sovereign.:thThumbsU
personaly im a gilson nut i have 1 but would love more just cant afford anything right now but i also love john deere , the older sears suburbans economy amf and im a closet fan of the craftsmanII and murray . there i said it im no longer in the closet lol