Mine was a police car. I remember getting it for Christmas when I was super little and feeling very excited when I finished it (with my dad's help) and could roll it forward.
I already had Duplo before this, but as far as I remember my first System sets were 7634 Tractor and 7636 Combine Harvester which I both got for my 3rd birthday (and built with some help). As for my experience, the Tractor was fine, although I was a bit disappointed in the harvester because it was so fragile and would break apart easily. I still have and like both sets today.