For me it has to be the steering quickener. That is a huge improvement over the lazy stock steering. I just recently installed a Pawltector e brake that is a good inexpensive upgrade also. No more slamming it into gear from neutral. This last weekend was the 1st ride with the Racertech medium springs front and rear. That is also a great improvement, not only more ground clearance but way better bottoming resistance and overall handling is greatly improved.