I just finished re-reading the full Dark Tower series, and enjoyed it much more than I did the first time I finished it. Always loved the first four books, but the last three seemed like something of a cop-out when I was younger. Maybe I had to grow in order to appreciate them fully.
Other favorites
The Sword of Truth, books 1-4. After that they get tedious.
Nightrunner series by Lynn Flewelling: Lesser known but excellent fantasy novels.
Xanth series by Piers Anthony: My favorites when I was young
Contact by Carl Sagan
1984 by George Orwell, and in a similar vein, though written more for younger readers; Little Brother by Cory Doctorow (read them in that order as the latter heavily references the former)
There are more, but that's good enough for now.