Jumping in with the sorta derail on ESB being best. I think most people say it's the best of the Orginal Trillogy for different reasons but subconsciously it's because the bad guys straight up win. Unlike most movies, ESB actually shows evil triumph over good. Rebels are soundly defeated on Hoth. Lando betrays them to the Empire. Han is tortured, flash frozen and zipped off to Jabba. Luke loses his hand and finds his daddy. And to top it off, 3PO is blasted(which may or may not be a bad thing depending on how much you like him) and Lando jacks Han's clothes.
Lando gets a bad rep in this movie. He betrayed Han, but it was either sell out one guy and his pals, or doom his entire city to Imperial control. As soon as he found out Vader was leaving troops in the city anyway and he had been lied to, he immediately went to the aid of Leia and Chewie. Lando's a good guy who can put the needs of a whole city over the needs of a few main characters.
Just like Han said, he's a real hero.
But in all honesty, I think that most of us would have done the same. It's easy to judge when you're only a spectator. And the deal got worst all the time. If anything, he was just naive to trust Vader at his words. The gambler got played.
