The Maxims of Manhood (sorry couldn't resist.) More seriously, I just read a book that I'm shocked isn't considered one of the 5 Greatest American Novels: All the King's Men. Effing blew me away.
Also, this is a terrific list of both critical and popular choices: 100 Best Novels: http://www.randomhouse.com/modernlibrary/100bestnovels.html
i didn't read Great Expectations like i was supposed to in high school but read it like 11 years later and loved it!!! really good book!
I should have mentioned The Book Thief by Markus Zusak. It's a crime that this book is classified as young adult. It's the story of a girl growing up with a foster family outside of Munich during WWII. It's definitely one of the best books I've ever read and is just a really beautiful story. Highly recommended.