No surprise... my absolute favorite part of Nintex Workflow, is User Defined Actions. Love them!!!
My most problematic part, is the lack of User Defined Actions in the Office 365 product. Once we implement that, it's going to be so awesome and open up some amazing workflow designs.
My favorite feature continues to be the State Machine! So easy to configure, and used for a variety of concepts.
I had to stop and think about a least favorite. But I wold say dictionaries.
It definitely is hard to pick just one. I love state machines and flexi-tasks. I can't think right off about one I don't like. I guess calling a web service. I guess that is one because one can't use it here because we don't have an account to call it. I understand the security and all be it restricts us developers.