My top 5 are:
- Safety bicycle - I'm glad I'm not riding around on a penny farthing!
- Pneumatic tires - Still the standard for a reason, despite recurring attempts to replace them with "flatless" options that tend to cause bigger problems than they solve.
- Freewheel - Sometimes coasting can is more enjoyable than pedaling.
- Geared hubs - The ability to change gear ratios, selecting from multiple options, during a ride was a game changer.
- (Weather-resistant) Hub brakes - Predictable stopping power in wet weather conditions made bicycles more practical.
Honorable mention: Spoked wheels, paved roads.
I am admittedly biased towards utility cycling rather than recreational cycling.