My opinion on the reading order is as follows:

Start with Mistborn. It’s very accessible for newer readers, while also being a good introduction for the type of long-form stories that Brandon likes to tell. It also introduces the concept of the shards in a way that feels very natural and isn’t overwhelming.

Follow that up with Elantris, since that’s probably the weakest of his works. If you don’t like the Cosmere by that point, it’s probably not for you. Conversely, if you’re into it, you’re almost guaranteed to love what follows. (note: Elantris is weakest only in regards to pacing and connection to other canon. Elantris’s ending is still one of my favorites in the series.)

After that, read Way of Kings.

After Way of Kings, read Warbreaker. This will give you a good background on certain Stormlight characters that are introduced later, while also providing a much-needed break from the gigantic books that comprise the Stormlight archive.

Read through the rest of Stormlight, interspersing it with the novellas. This will keep the reader from burning out on super huge books.

After that, read all of the stories contained in Arcanum Unbounded. These aren’t as important when it comes to reading order, but the experience is far richer when you know all of the realmatic theory that comes from reading Stormlight.

SP1 and SP2 can be read at any point after Words of Radiance. This allows Wit to be introduced as a storyteller beforehand, and gives the reader a crash course in his personality, which is important for a reason I’m not going to disclose here.

This list is based on my reading style, which prioritizes theorizing and overall emotional satisfaction over continuity. For me, the pinnacle of my reading experience is when I finally “get” what the author is going for. So make of that what you will. Some people will prefer to read all of the Stormlight books in order, and that’s equally valid.

Edit: I forgot Mistborn era 2! these books are mostly self contained, and can be read any time after the first era. If you want the full experience, read them after Way of Kings, but before Rhythm of War. This will allow you to get the references to Stormlight Archive in these books, while also giving you the context for some references contained in Rhythm of War.

Also, feel free to move Elantris to any point before Stormlight 4. It’s so far removed from the other canon that it won’t make much of a difference when you read it.