In my opinion, Skyward Sword is not the easiest Zelda game thus far. It has a great variety of challenges, most of which are
straightforward, not
effortless, and I pretty much enjoyed them all. The only times that I found them overly-annoying and laborious were during the repetitious sequences of the The Imprisoned and the Silent Realms. Otherwise, they were simply concise and to-the-point. That's how I preferred them, because they were moderately difficult while staying away from being way too easy to beat.
For one thing, it presented a myriad of puzzles throughout the entire game, which were all solved using Wii Motion+ technology (read
this post to see my thoughts on that). It was and is deeply satisfying to at last use pretty much all the weapons available in my arsenal in order to get through Dungeons or just the provinces. Even the enemies themselves became puzzles, requiring us to look for a way to outwit their defensive stances. Therefore, that to me is tangible proof that Skyward Sword is not the "easiest" since it causes us to think about our movements before we react. Additionally the gameplay is smooth and non-gimmicky, not oversimple to the point that it is boring to death. That feeling simply comes from some of the objectives that we're forced to deal with; such as, again, the Silent Realms, The Imprisoned, and even the Tadtone hunt.
In terms of combat, I'd say Twilight Princess is the easiest (especially since it's assisted by the Hidden Skills), and for puzzles I'd take that title to Wind Waker, because half the game is made up of sailing. [Not bashing either of them; just sharing why I think they fit it more than Skyward Sword].