My "best"?
For O scale, I guess it would be this tired old Varney/All Nation Ten Wheeler:
As I posted in the 2 rail forum, when I got this the motor was seized up tight, some valve gear parts were missing, and one of the center drivers was just a little loose on the axle, causing a fairly serious bind.
Thanks to some of the guys here, I was able to get it up and running again; and it does that very nicely, I might add. A good learning experience and a project that gave me many hours of pleasure.
Dunno if it's my "best", but I like the fact that I had to put some time and effort into getting it road-worthy again...
Mark in Oregon