We can only pick one? Awww, that's no fun...
Salvatore's Drizzt series (all of 'em except Spine of the World, the one that focused on Wulfgar) is good; I've read most of them twice, some of them three times. Looking forward to the continuation of the series.
The War of the Spider Queen is also great, and has a chance of becoming a favourite.
Weiss/Hickman's Dragonlance: Chronicles and Legends trilogies. Also enjoyed Death Gate Cycle and Star of the Guardians, both soloed by Weiss, IIRC.
Celia S. Freidman's: Coldfire Trilogy. Sweet!
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Duh, how could I forget Harry Potter! It's cute, fun, and quick. Love the books.
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However, to narrow it down to one series, I would have to choose the first Fantasy books that I ever read, the ones that got me addicted to the genre and the D&D game...
Raymond E. Feist's "Riftwar Saga."