What is the most valuable higher-end cycling gear you can buy to improve your riding experience?

To improve your riding experience after the frame, tires and gearing (shifters and gear train) are typically the most valuable upgrades.

Tires have a huge impact on comfort, rolling efficiency, and grip. High-quality tires can drastically change how your bike handles, especially on different surfaces.
Gearing (shifters and gear train) affects smoothness and precision in shifting, which directly influences performance, especially on varied terrain.
Other upgrades like brakes or seat can improve safety and comfort but generally have a smaller impact on overall performance compared to tires and gearing.