#2 Benfica
Having won the old European Cup twice in a row back in 1961 and 1962, Portugal's most successful club, Benfica, were looked upon as a club that was going to win many more of those trophies in Europe.
However, since then, they have failed to win a single European competition in all these years. There is also a theory that says that Benfica's legendary manager Bela Guttman's curse is the reason behind the club's underachievement in Europe.
After helping the club win those two trophies, he requested a raise in his salary but it was turned down by Benfica's management. When he left the club, he is known to have said that Benfica won't be European champions in a hundred years and over the next 55 years, the club have failed to win anything in the continent.
Benfica's underachievement is not down to fundamentally poor football since, after all, they are Portugal's most successful club. In fact, they have reached the European Cup final five times and the UEFA Cup (Europa League) final thrice. However, they have not been able to cross the final hurdle.
They lost the Europa League final again in 2013 and then did the same in 2014 to continue their heartbreaking pattern of losing European finals.