The absolute best: New set of tires with the right casing, tread, and compound for your riding.
The worst: Spending money on any part or accessory other than the above if your tires are worn out or not the correct casing, compound, and tread for your riding, even if the current set are brand new.
If you have a brand new set of correct tires, then purchase large platform pedals with long metal pins (at least 5mm sticking out, the hardware store is a great place to get new pins) and five tens (if a flat pedal rider). An alternative is purchasing brakes that don't require you to squeeeze them to lock up.
P.S. No part or accessory purchase will do more for your riding than a long weekend at a bike park or shuttle area with people faster than you. It's amazing how many places you'll notice yourself braking or turning unnecessarily when riding behind your "fast" buddy and he starts pulling away in the first corner.