The easiest Has to be either Minish Cap or Phantom Hourglass.
Minish Cap is very linear (that's not to say it isn't good, it is definitely in my top three Zeldas and was in fact the first I ever played.) It is VERY difficult for you to get confused and not know what to do next, all you ever have to do is ask around and you will discover with ease where you need to go and what you need to do, there are never any riddles or even clues it's pretty much just standard "I heard there is a ______ in the _____." Then again, parts of it could be seen as mildly challenging, the temples or Vaati or any of the bosses infact. On top of that The cuccoo mini-game is still in my opinion one of the hardest there is (it gets me VERY angry
.) I think part of the ease of this game though might just be that the playing area is so small in comparison to other games, thus making it near impossible to get lost or to fail to find something.
Phantom Hourglass is much less linear (though still rather much so) you can seem to actually choose where you want to go next at parts and explore as oppose to having to do everything in a very particular order. The general gameplay is much easier though, all the minigames have low high scores or easy time limits, and the fact you play with heart containers as oppose to heart pieces where in fact we could have had a lot of fun trying to find all these heart pieces on tiny islands at sea instead there are very very few heart containers of which i believe next to none are actually hidden.
I see alot of people say that Twilight Princess is one of the easiest, but I have to disagree purely because of the sheer size of it. By that I mean the size of the Hyrule as well as the actual length of the game. The size of the Hyrule makes it all too easy to get lost, or to not know where to go where as in some other games as I mentioned above, that is much easier. Not only that but the length of the game, for example the two hour temples, can make things very challenging. I got this game for my birthday a few years ago, and my birthday is in the middle of exam period, thus it was very rare I would be able to dedicate 2 hours of solid playing as I spent most of my time in exams or revising *cough*.... Anyway point is I can remember several occassions where I would have to stop halfway through a temple and save, and let me tell you, it can be VERY confusing to try and find your place again.
I think that the Zelda games made specifically for handheld consoles are generally easier as there is just much less space in the game for megabig maps or temples.