All the Cosmere books are really good. Warning though, Stormlight is about to release book 3 of 10. Mistborn is also actively being written, but the next book to be released is 4 of 4 for "Era 2". (the eras being self contained but building on the previous)

Elantris is the least well written, so read it first or not first depending if you want to lose interest in Sanderson, or be let down by the writing style. The rest are excellent though.