Depends on the person really. If I had to choose an easiest it would be yes / no questions to the cosmos using pendalum. If I had to choose a hardest I would say scrying with a black mirror. Just my opinion though.
I would have to agree with MindlesChaos. Although I would broaden it to dowsing being the easiest form of divination, with scrying the hardest.
There is little prior experience needed for dowsing, and it's pretty straightforward. In it's most basic forum the essential idea is "if the object does this movement, it means yes. But if it does this movement, it means no."
Scrying on the other hand is far more complicated and in most cases requires a great deal of focus and discipline. You are after all staring into a object while in a trance, waiting until images begin to form in your mind's eye. It's a very lengthy process compared to dowsing, which can be done in only a few moments.
I have compiled a list of divination methods (as well as other psychic related terms) into an article I published here: