I'm partial to the whole package. To me no component stands alone. But if forced through torture to pick one, I would say wheels. It amazes me that something seemingly so fragile and light can take the punishment they take. But then it comes back to the whole bike. I am just fascinated by the fact that we have successfully developed a conveyance that efficiently moves us from place to place that needs no fuel, oil or hay and weighs in at below 20% of my body weight.