Football comes top by a long way. I don't know which sport comes in second place but nobody has mentioned golf and athletics yet. The PGA European Tour goes from strength to strength each year and most of the big IAAF Diamond League events are held in Europe. The Wikipedia entry for 'Sport in Europe' claims that ice hockey is the second most popular sport in Europe but doesn't provide a citation.
It's an interesting question because it raises the further question: How do you measure how popular a sport is? Is it based on the number of playing participants, the number of spectators, or both?