Wow Jim, that is really a tough question. At one time I was really hooked on the 3444 cop and hobo. The PRR TMCC Crane is near the top of my list today, but any of the freight cars out of the 1631WS set are still at the top. My father bought us that set when I was 4 or 5 years old. Between the 6519 Allis-Chalmers car and the 6812 Maintenance it would be hard to pick. We destroyed that set growing up but I was lucky to get a steal on a near mint set of of E-bay 10 or more years ago. Train looks like it was ran once and put away in a time capsule. I bid $450.00 dollars but got it for I think $225.00. When you ask us what our favorite set is you have the answer here first. Guess it wasn't that tough afterward.