I'd be hard-pressed to limit myself to just one series. Series worth considering:
LotR (Tolkien) - Obviously.
Riftwar (Feist) - Still am not motivated enough to finish books 3 and 4 of the plodding Serpentwar series.
Mars (Burroughs) - An old-time favorite.
Mars (Robinson) - A mucvh newer favorite. I devoured these.
Belgariad (Eddings) - The Great Bubblegum Fantasy Series. Lots of fun. Emphatically not including any of his other series.
Earthsea (LeGuin) - Great series, astoundingly well-written.
Hyperion (Simmons) - Great sequence, especially the first two. I've gotta re-read these soon.
LeGuin is probably, I think, the greatest living writer of F/SF.
Elric (Moorcock) - Not incluyding the much more recent additions to the series.
Foundation (Asimov) - Original trilogy only.
Conan (Robert E. Howard
only) - I am a Conan Nazi.
Gor (Norman) - Ha! Just kidding. Still, better-written than Dragonlance.
If I
had to pick one series, it'd probably be Conan. Or maybe LotR if I could include the Hobbit, Unfinished Tales and all the other ancilliary material - you could spend a lifetime going through that.
Now, if I had to pick one
book, and one book period, it'd be Roger Zelazny's
Lord of Light, my all-time favorite F/SF novel (yes, even more so than LotR or Dune.)