Pride and Prejudice -- hands down! I love the language and wit with which is written, and well as the variety of romantic entanglements that Austen creates. It is great satire and yet says something important about what happens when pride and prejudice get in the way of how we interact with others. I am also a sucker a romantic happy ending.
Oh, by FAR it's The Great Gatsby. It's just a wonderful story, full of debauchery, that is set in a definite time period (the roaring twenties). Just studying the 1920s alone is amazingly fabulous with all of the fads and the folly. Add to that an author (Fitzgerald) who was as obsessed with a woman in his own life (Zelda) as Gatsby is with his (Daisy), and you get a winner! Hence part of my screen name: Ms-CHARLESTON-yawp. (The charleston was the big fad dance from the 1920s. I'll guarantee that it was danced at Gatsby's parties.)
For me, I think it would be The Adventures of Huckleberry Finn. Comparing it to some of the ones that you've mentioned, it has a good story like Oliver Twist but is a more familiar setting to me as an American so I understand it better. It is more fun than Scarlet Letter and The Great Gatsby because those, to me, don't have as interesting of plots. So I like it because it's a nice mix of classic literature and an engaging story line.
My favorite classic novels are all the novels by Charles Dickens. There's something about the stories in connection with the Industrial Revolution at that time. British was in a very dark era when Dickens live, when he began to write about his country, every single detail and the situation are perfectly described. We can learn so much about that era, the social life and interactions, even the politics and legal system. He's simply a genious.
I must say Pride and Prejudice. The characters created are so vivid and real. Elizabeth Bennett and Mr. Darcy are the best love story - two people who find love with each other against the odds. I also appreciate the irony of the prejudice by which Elizabeth believes Mr. Darcy guilty of but in fact she is guilty of the same offence. I read this book at least once a year. I introduced my daughter to Jane Austen when she was a teenager and it has also become a favorite of hers.