Mistborn is still the best place I think. It's average length for him and really shows his style of writing (even if he has improved over the years). If you like this, he is for you. Tress of the Emerald Sea is great to start with as well. Only thing against it is that it's quite different from how he usualy writes. The Emperors Soul is only about 200 pages and his best work in my opinion. It totaly stand alone and while short a bit slower in the storytelling compared to his other books.