It would be unfair not to post the names of series that I found thoroughly entertaining as a young adult:
-The Chronicles of Narnia
-The Seventh Tower
-Animorphs (A SPECTACULAR juvenile series, really, the way the whole universe is crafted is marvelous)
-Deltora Quest
-The Artemis Fowl Series
Runner-up to grand prize, of course, is The Lord of the Rings.
BUT the top slot will always and forever remain Harry Potter. Never have I been more completely captivated by a series - for four years, literally the only books I would read would be re-reading the HP series, and this is four years counting AFTER Deathly Hallows was released. Truly, remarkable plot and relate-able, awesome characters.