I have long been a fan of the great David Eddings.
The Belgariad (5 books) and the Malloreon (also 5 books) are incredible epics of modern fantasy writing. Critically acclaimed bestsellers, and adored by his readership, these are truly books that you HAVE to read at some point in your life. In addition, the follow up books (Belgarath the Sorcerer and Polgara the Sorceress) are both amazing stories that serve to increase the pleasure of re-reading the others. Athough not a story per-se, The Rivan Codex (a collection of stories and authors notes from the series) is a must for anyone who is truly a fan of this series.
His greatest works (IMHO) however, are the Elenium and Tamuli trilogies. The Belgariad and Malloreon are Harry Potter-esque in that they follow an orphaned boy through the discovery of his magical powers and the fulfillment of his destiny (interesting to see how Ce'Nedra and Ginny, and Garion and Harry seem to share a few personality traits - there's a project for you: compare and contrast the Garion/Ce'Nedra relationship with Harry/Ginny, fully exploring the personality traits and comparing Cho to Zubrette

.)
The Elenium/Tamuli however are more adult books, focussing on older, more down to earth characters, and being a lot more plain speaking in their tone and humour. These are Eddings' greatest work so far, and are a must read for any fantasy fan.
Runechild.