Wheels & Tyres
A change of wheels and tyres can transform any bicycle. Smoother bearings and reducing the rolling weight help with acceleration and hill climbing. Deeper profile rims aid with aerodynamics and speed. If you're looking for a better feel from your bicycle, the wheels are the place to start.
Tyres also make a big difference to the ride. More grip, less resistance, better puncture protection, a new set of rubber on your wheels can also help reduce the weight, again resulting in a quicker accelerating bicycle.