A box is the easiest and the one that takes the least skill to make sounds good enough to bring in a turkey. Get a decent one and you're all set (if you don't want to spring for a custom call, something like a Primos Box Cutter will do you just fine).
I'd say next easiest is a pot call but striker angle and pressure can contribute to good sound or lousy sound. And conditioning can be a contributor to poor sound. You can mess up on them...
Not that these two take tremendous amounts of skill to make "good enough" sounds because they don't. Don't be intimidated by any call. They all can be mastered with a little practice and minimal instruction that's available from a friend turkey hunter or on the internet.
