I love all of you who said "The Great Gatsby". I've always loathed this book and had to read three times in high school. It got worse each time. What is the point of that book, exactly?
Anything by Nicholas Sparks. Maybe it's because I'm an aspiring author, but I hate, hate, hate when authors reuse the same storyline over and over again. Changing the names doesn't mean it's a new story!!!
As far as underrated books go, I read mostly Christian fiction and my favorite books of all time are written by Liz Curtis Higgs. The first one is "Thorn In My Heart"... I reread these books about every year and *every.time* I get so emotionally involved that I bawl/laugh/throw the 2nd book across the room. Wonderful, wonderful writing.