I think the best visual medium for epic fantasy is television, when it's done right. Has anyone else been blown away by the trailers for Game of Thrones on HBO? I would love to see that same kind of treatment for, say, the Wheel of Time.
Well, epic fantasy is very, for lack of a better term, long. Any other medium that can stretch so long could work. So Tv does, but only if you get enough episodes to do it right. Graphic Novels and Audio can (and do) also work. I would love to see a Stormlight Archive graphic novel with enough room to do it right!
The problem with TV is that if it's to be done right, episodes get expensive, so there is often strong pressure to cut a lot of material. In the case of Game of Thrones, which has turned in my opinion into 90% chaff, that can be a good thing
Side note: It called the freaking Song of Ice and Fire! Spoiler Alert, Jon and Daenerys will save the world and most everything else will be mostly irrelevant. Duh! To use the architect versus gardener analogy, Game of Thrones was an incredibly fertile ground, but it's now overgrown with a lot of weeds... /rant