As far as series are concerned:
It's a 3-way tie between
Lord of the Rings,
Harry Potter, and the
Belgariad.
It's been a long time since I've read
G. R. R. Martin, but I was disappointed with the fourth book in his recent series (Fire and Ice?) and never did finish it. The stories didn't seem to be going anywhere at that point and he killed off characters I had grown attached to.
I used to be a big fan of
Michael Crichton (read most of his novels) but his recent books have been too preachy. I think, maybe, I've just outgrown his style.
And, as for the Hitchhiker's series, I believe I read all the books that had been published up until I got out of high school (that may just be the first four), and that was my favorite series for a while.
Not anymore, though. Now's it's definitely the 3-way tie between the greatest fantasy series that have ever been.
