I prefer the Nexus devices. I like Android the way it was designed. I don't usually like when the manufacturers try to "make it their own".

You have Iphone's awesome UI which focus very much on Usability but lacks multitasking ability
Samsung's Touchwiz with great multitasking ability but lacks on the usability department which many complain ... I find it easy to use though
Then you have Nokia's Window UI which I really love but it has a lot more room to improve
Emotion UI from Huawei Edge, much more improved usability in compare to samsung but probably lacks features
Then the HTC and Blackberry UI

Which one do you think is the most sophisticated to the point it frustrates you or one that you enjoy using the most?