For me Cubase was the easiest to learn. Pro Tools is right there as far as ease of use goes though as well. Sonar was OK, Digital Performer was pretty easy, Logic was harder, Samplitude was OK etc... In the end though, my views are also a bit biased because I have by far spent the most time in Cubase as opposed to the other applications. Have you considered getting a book that help to teach you how to more efficiently use Sonar? I was doing pretty well in Cubase, but after reading Cubase SX Power things were much smoother and more efficient for me. I knew how to do a lot or most of what I needed, but many things that I was doing were not being done in the easiest and/or most efficient way.
The real problem is that no one can really tell you which is easiest. What is easiest for me may not be for the next person. A lot of it depends on personal preference, workflow, prior understanding etc... as well as basic recording knowledge. All that being said, I like the Steinberg apps the best, and sadly, I would place Pro Tools next.