Personally, I'd like to add at least Stephen Donaldson, Gene Wolfe and Roger Zelazny to the list of authors everyone with a fantasy interest should read. Stephen Donaldson's fantasy is the kind that eschews Tolkienesque staple settings for his own (though with very strong Christian overtones), in the Chronicles of Thomas Covenant, the Unbeliever or in the Mordant's Need series, we see protagonists who don't believe in what they see and where they are or the narrative from the perspective of those they meet, and there are consequences... But his real gold writing is in the short story books The Daughter of Regals and Other Tales and Reave the Just and Other Tales. I haven't read his Gap series, so I can't speak about that.
Gene Wolfe is simply incapable of writing less than top notch. If Science Fantasy is admissible for this thread, I highly recommend his The Book of the New Sun. Highly under appreciated author who deserves more credit.
Roger Zelazny had one really famous fantasy series written by the time he died in 1995, The Chronicles of Amber. It's two five book series from the time books weren't as thick as they are today, and you can find the entirety in one very thick volume if you search for it.
Some authors that are already mentioned here.
- David & Leigh Eddings:
Sure, the Belgariad was my first forage into the realm of original language reading and as such I have some nostalgia for it. But it's not their best work. The real problem with their writing is that it's highly repetitive, motif wise as well as actually repeating same events from different perspectives. As such, I think a single book of theirs is actually a much more enjoyable read than any of their series, that book being The Redemption of Althalus. Though if you have a young teen and want to introduce them to fantasy, I think the Belgariad is a good choice.
- Robert Jordan:
The Wheel of Time has many a fault as a series. But one thing this series has that most fantasy books gloss over, however, is a really fleshed out magic system which is described to a degree that you could probably build a computer model of how it works. Not really a good system for a game, from my tries, but a good cohesive model for magic.
- Raymond E Feist:
I agree completely with the notion that the interesting thing that keeps you reading these is the recurrent characters and real felt time progression. However, I don't think he is as good a wordsmith as most of the authors mentioned here. There is one book I return to from time to time though, and that is Magician. Which he incidentally considers a mistake of his youth and inexperience.
- Terry Pratchett:
Discworld is hilarious, I see it as a fantasy equivalent of what THHGTTG is to Science Fiction. He's got other books though. See if you can find his Carpet People.
And a couple of authors I just want to throw out there:
- Orson Scott Card. I haven't read much of his, but I have read the Tales of Alvin Maker. He's worth a read.
- Terry Brooks. His Shannara books might be a bit Tolkienesque, but if you ask me, this is a series to read for the villains and monsters.