"Classics" I loved:
This Side of Paradise (my favorite book ever -- I like it a thousand times more than The Great Gatsby -- and Fitzgerald's short stories are even better)
A Tree Grows in Brooklyn
1984
Brave New World
To Kill a Mockingbird
Pride and Prejudice
The Faerie Queene
"Daisy Miller"
Lolita
anything by Dorothy Parker
Classics I hated:
Paradise Lost
Great Expectations
Hawthorne's short stories
As an English major, I've found that the literature I'm lacking in tends to be the stuff that was originally written in Russian or Italian or French or what have you -- Anna Karenina, Dante's Inferno, etc. -- since most of my classes are English or American lit classes. Also, since I spent most of my childhood reading junk like Sweet Valley High, I missed out on a lot of children's classics like Little Women or Peter Pan. I went back and read a lot of classic children's books this summer
and I would definitely recommend that too -- most of them are just as good and more interesting than the "grown-up" classics.