Its hard to ask advice which engine is the easiest to use/learn, because the easiest one will be the one that makes the most sense to YOU.
Basically if the way the developers set up an engine is similar to how you would do it yourself, it will be incredibly easy to learn and use for you, but not necessarily for someone else. If a highly trained and experienced software engineer were to use an engine like C3, it might be very slow (relatively) for them to get what they want done! On the other hand, for me the flow of logic and events in C3 behaves very much in line with how I would have expected it to, therefore it was very easy to pick up.
The best way is to try different engines for yourself and see how far/fast you get with each one. It should be pretty clear after a day or so of experimenting.