Tad Williams is a very good author. His "Otherland" series is one of my favorites. It's a weird blend of Sci-Fi and Fantasy in that people get trapped in a computer simulation of various books and fantasy realms. One is a sort of Cold War Oz, one is World War One but with a giant beanstalk, and another is a weird kind of racist old cartoon. In addition to what's going on in the Otherland side of things there is a whole group of characters that are doing things in the Real World and their story is just as exciting as the people in the computer simulation. It has a weird ending, and the series is four books long, but I really liked it.
For Epic Fantasy you can't go wrong with Tolkein, he really changed the way people thought of fantasy, bringing out the epic stories that others only hinted at.
His legacy is carried by several authors. Of the people that write epic fantasy now I'd say the ones that are the best at it are George R. R. Martin and Brandon Sanderson.
George R.R. Martin wrote "A Game Of Thrones" and it's sequels, and they are amazing. They are also very long and very complex and at times not super exciting. Don't get me wrong, they're all amazing, but there are times where he's setting up political subplots that are amazing when they culminate in these amazing backstabbings and betrayals, but the build up is a little... less than thrilling sometimes. Still, amazing books, but very adult. Lots of swearing and a fair amount of sex and violence.
Brandon Sanderson also does epic fantasy, and he does it really well. He has a stand alone novel called "Elantris" which is fantastic. It's about a fantastic city where some people were basically The Chosen and could perform a sort of magic by drawing symbols in the air. This all changed one day though, the magic stopped working, the city started to instantly age and The Chosen all became wretches, leper like and lost all the power that they'd been granted. It's a great story with a really interesting system of magic in it. The heroes are very heroic and smart, all of them are smart enough to seem like real people, they put together plans and figure out clues and discover secrets that are too important to the plot to reveal. If you're looking for a good starter on Sanderson this is the place to begin.
Sanderson also wrote the "Mistborn" trilogy, which is a fantastic fantasy series that starts with a book where a team of thieves sets out to steal the one thing the God-Like Emperor wouldn't expect them to steal: His Empire. This is an amazing series and has some of the most interesting fight scenes I've ever read. Characters leap around, almost flying because of their kind of magic like abilities, and there are some amazing fight scenes that all happen in mid-air.
And if you're looking for truly epic from Sanderson you've got to check out "The Way Of Kings". It is book one of a series of which he is planning 10 books total. The thing is: It's amazing. It's almost impossible to describe. There are magic swords and magic suits of armor and there's this war with a race of people with marbled skin, and there's giant insect like creatures that have Gem Hearts, and there's a warrior/slave who used to be a doctor and it's all crazy complicated and good.