So for those who don't know, many of Brandon's books are very separate from each other on very different worlds with different magics but are set in the same overall universe. You might have metal based magic in Mistborn, or storm/oath based magics in Stormlight Archive, but each are pretty different. You don't need to have read any of the other series of books to know what's going on, but there is some crossover between a few characters that can hop to other worlds that have very little bearing on the story but are fun to know.
The Mistborn Trilogy would be a good interesting start, or perhaps just do Elantris. The Stormlight Archive is simply too big and too slow to do I think (and well, is only half finished)
As he's getting older and has found success, you can tell his writing is starting to suffer - I don't expect we'll ever get an answer to the whole Cosmere storyline, and I don't think the TV shows will ever manage it either. There just isn't enough time to do all the different series before an executive decides "Lets reboot this one". But that's ok, as long as we get a couple of series out of it, I'll be chuffed.