I use the Palmer Hoot Tube also and love it. Though I don' t think it is too easy to learn, not hard mind ya' , but not the easiest.

Primos Camp Owl is the easiest I have used and also sounds great. It' s actually two owl calls in one. Camp owl & Power Owl. You can have nice soft calling to medium loud with the Camp barrel on the Power Owl, then if you want to reach out there and touch someone you just the Camp barrel off and use just the Power Owl. This call rolls very easy. Also Doug Camp makes the Camp Owl (original version) which is wooden and sounds great! Also very easy to use.