Hmm... Tough question.
I've been immersed in the Discworld series for a long time, so it's definitely near the top of my list. Whether it's number one, I can't say for sure.
I'm also a huge fan of Stephen King's The Dark Tower series, which I've read through to completion (seven books) about four times. I actually just recently discovered the graphic novels of the series, and have been tracking down those, but, honestly, I really don't like paying around $30 for books, so I've only got a couple.
Those two would be at the top of my list. And, of course, it would be wrong of me not to mention my love for all things Tolkien, and how I enjoyed A Song of Ice and Fire. I suppose I should also give honourable mention to Margaret Weis and Tracy Hickman's The Death Gate Cycle series, though I've only read three or four of those, and not for a long time. But they were a big influence on my love of fantasy when I was younger, along with Tolkien, so they deserve praise.
Other than that, I suppose that, given my age, I have to mention the Harry Potter novels, which, while rather poorly written (in my opinion, anyway), told an excellent story, save for a few stumbles here and there.
So, yeah:
Discworld
The Dark Tower
The Lord of the Rings (and really anything Middle-earth related)
A Song of Ice and Fire
The Death Gate Cycle
Harry Potter