Oi! I love Warbreaker! Best single book in the Cosmere, in my own opinion.
For series, though, The Stormlight Archive is my one true love. I don't think Mistborn even comes close after the first book ends. Then again, Mistborn is a little dark for my tastes, personally (and I'm speaking to subject matter; I find certain aspects of the magic system deeply disturbing).
For other places to start, once you've finished Mistborn, you may enjoy Tress of the Emerald Sea. Some people might recommend reading more of the Cosmere first; however, I think it's a great place to get into things because it'll leave you with some questions about the wider Cosmere while still feeling like it's own book. Otherwise... Stormlight. Always, always Stormlight.
On another note, if you could choose any character (from the Cosmere books you've read) to be your college room mate, who would you choose and why?