Probably gonna get crucifed for this one....
3001 by Arthur C Clarke.
I don't think I got past the 3rd chapter. It's like he threw something together in a hurry, to make a dollar. The first two books were pretty good, 2061 was okay at best, but I just couldn't finish this one. Just too weak and unreadable for me. I don't even know how it ended, but when I stop reading after chapter 3, that was too much.
Honorable Mention-Kevin J Anderson's "Fantastic Voyage" of an alien body(I've forgotten the exact name). I say honorable because it was totally readable until it disintigrated into another "Aliens want to take over the world" hokum.