I nominate my 1966 Massey MF10
I got this tractor a while ago bought it unseen for parts, when I went to pick it up discovered some different features and after researching it found out that it was first year production for the MF10s, 12s. Decided that it was one to keep and fix up for myself. The more I got into it the more I found out about it. So it was tested to find out what was good and what wasn't. Then disassembled, cleaned, and fixed whatever I found wrong with it.
One part that was broken when I took it off was the steering wheel and to this day I still do not have a really good replacement one for it. Another part that I wanted for it was the aluminum nose ring that came on the early 66 models, that part I was able to find thanks to a fellow GTtalk member.
It was all painted and new decals put on and is now a good working tractor. I have an engine rebuild kit for it to be put in shortly, the engine runs OK just has a rod knock in it. Also I have 2 sets of narrow rear wheels for it, turfs and AGs. This tractor will probably get babied alot as it's the nicest one I have, but it is capable of doing what ever they could do when it was brand new.
There is a thread and an article on this tractor here at GTtalk, tells the full story.