Cycling because they got the two Golds. Rafa Nadal dominated in tennis too. Those would be the best. They also got Golds in canoeing and sailing. If I was to rank their top 5, it would be.
1) Cycling - 2 Gold, 1 Silver, 1 Bronze
2) Canoeing - 1 Gold, 2 Silver
3) Tennis - 1 Gold, 1 Silver
4) Sailing - 1 Gold, 1 Silver
5) Synchronized Swimming - 2 Silver
They also won Silver in men's basketball and men's hockey.