Originally Posted by Jughed

Depends on your starting point. What are you currently riding?

Road bike? If so:
-modern 28-32mm race tires, like GP5000’s or similar, are fast, supple, light, grippy and puncture resistant.
-wheels, lighter, deeper section wheels can change the entire bike.
-wide range rear cassette, something that will get you a 1 to 1 gear ratio or better for climbing.

IMHO, the best bang for your buck performance improvements.