The Empire Strikes Back is known as the darkest movie in the series at least until Episode III comes out. A lot of hardcore Star Wars fans would say that ESB is the best of all the films. When Vader told Luke "I am your father" that's what made ESB the classic that it is. The light saber duel between Luke and Vader in ESB in my opinion is better than the one in Return of the Jedi. The Empire Strikes Back has lots of action and it has Boba Fett! Unlike the other movies the big battle in ESB is in the beginning with The Battle of Hoth. After Hoth, Han, Chewie, and Leia head to Cloud City where they find Darth Vader. That's where Han is frozen in carbonite and that's where the battle between father and son takes place. The Empire Strikes Back is a great movie and in my opinion the second best movie in the series.
I would give The Empire Strikes Back a 9.5 out of 10.