I suppose in terms of sales it might have been King Kong there for a while.
AKA the Z50, AKA the “Monkey Bike”. I started on a Z50 myself about 40 years ago. Gave the bike to a friends little brother at some point. Last I heard it was still going strong. Those tires were a PITA to change.
Motorcycle Photo Of The Day 
My first motorycyle was a 1986 Z50OR in the 90s. You aren’t kidding about changing tires. I tried every trick in the book to make them thornproof.
that little 4 cycle would start first kick and you just could not stall it. My ~280 lb father could ride it even. It started in cold weather or hot and just plain worked.
The newer versions have taller skinnier tires with more suspension travel and are a much better bike IMO. Further they will take longer for your kids to outgrow.
Ive heard and always believed that’s he most produced engine in the world. then add the 90cc which is a derivative,
Loved readding this thank you