I personally started on Stormlight (it got recommended to me by someone who had only read it) and I would say it can definitely get dark, especially:
Warbreaker absolutely has some scenes I can think of despite not having read it for a long time, but I would probably agree that Mistborn is the toughest or darkest of his work.
On this question, the fun answer would be someone like Lift, Wayne, or Hoid, but I'm not a party person. So I would probably say Adolin (I'd like to think we would be great friends and he could pull me out of my introversion) or Sazed (how much fun would it be to talk about philosophy or theology with that man?!).
