Yes this is a fun question.
Thinking back, probably the old Monogram Phantom Mustang. For those not old enough to remember, it was a well detailed airframe with a clear skin. It sat on a red control stand, and I believe one could make the prop spin, gear retract, and maybe something else I forgot.
I played with that thing for hours on end, and it might be what made me fall in love with aviation. Yikes, that kit cost me a lot of money in hindsight!!!