For me it's the legs. I just started cycling, but have been rowing on a stationary ergometer for years, so my cardiovascular conditioning is pretty good. The legs are what do me in. On a really steep hill, they start to burn and gradually begin to feel weaker and weaker as I pedal. Eventually, I stop. I could keep going but for them. However, the legs seem pretty easy to train. It takes a long time to build up cardio fitness, but my legs seem to be getting into shape pretty fast.