"The first and the best. It was originally just one volume--broken up by the publisher into three books. It is still the standard for High Fantasy, Epic Fantasy and Fantasy Trilogies--Introducing such elements as the Quest, the ultimate fight between good and evil, world-building complete with unique languages, maps, and non-human races: elves, dwarves, halflings, etc."
"The Fionavar Tapistry. Guy Gavriel Kay is one of the best writers of fantasy, ever. His roots go back to Tolkien: He was asked by Christopher Tolkien to help out with his fathers notes which were subsequently organized for _The Silmarillion_. Kay's first original creation was this trilogy, which still holds up over time. I would highly recommend his subsequent books, too."
"The Gormenghast Trilogy has long been regarded as a classic of literary fantasy. It is a bizarre world--a gigantic city/castle, Gormenghast, populated with strange characters that owe more to the stranger aspects of Lewis Carroll than anything else."
"This is the first in a trilogy, of course, not Epic fantasy, as the previous, but a more character-driven tale, of which this is an early and good example that has influenced many similar tales that have come afterwards."
"The Deryni Chronicles. Another early character-driven trilogy featuring magic-users who are distrusted, set in an kind of alternate history. Again, featuring elements that have been used many times since in fantasy novels--but this was one of the first and still one of the best."
"The Wraeththu Chronicles: Enchantments of Flesh and Spirit, Bewitchments of Love and Hate, Fulfillments of Love and Desire. A trilogy, poetically powerful, character-driven, of a post-apocalyptic time when humankind is failing and a mutation creates a new race of beings with an alternate physiology and magical powers. It weaves in elements of the early punk/goth culture and sexuality."
"The Coldfire Trilogy. Here we have the ultimate in strange bedfellows, with a Paladin-type having to pair with an Evil Sorcerer in order to fight a greater evil. Moral ambiguity is further explored... and our own fascination with conflicted characters."
"Kushiel's Legacy. An alternate Europe with alternate religions and history. A good dollop of sex (fetishes) and romance, but with all the other elements--great world-building, great characters, great intrigue and great battles for the love of home and country."