I think I am an easy client in that I am always kind and respectful; I don't get angry very easily and I never ever yell or lose my temper. I do listen and I make a big effort in therapy.
But I am a bit difficult in that I have some serious trust issues and it takes me a looooong time to open up to a therapist and I sometimes lie because I'm too apprehensive of the therapist to tell the truth. I'm sure that can be frustrating -- but as others have said, the "difficult vs. easy" factor in this case largely depends on the T.
When I terminated with my old T, she said she enjoyed working with me and that she appreciated that I never skipped a session. So there's that.