This was hard to pick just one. The first tractor I was close to as a child was a John Deere two cylinder diesel with a gas starter engine. I remember seeing them pull out an old hedge row on my grandparents farm with it. It dug foot deep ruts in the dry pasture pulling the stumps. I loved to listen to the sound of the engine under load. I kept waiting for it to stall, but it never did. I was amazed at the power considering it sounded like it had a dead cylinder. So I guess if I were to pick one, that would be it.
Mark