WW; the Nintendo gallery is probably the most tedious thing to do. However, for someone like, who likes to gather information, it's perfect. Having to constantly go around the Great Sea though then you have to come back to the gallery with three pictures, then you have to turn the time from night to day or vice versa. Then there's also the heart pieces, which are scattered just about everywhere, and you have the treasure charts to look at too. You can spend hours looking for everything. It's not really hard though, so much as tedious.
I guess I would have to say MM is the hardest. This is mostly because the game emphasizes the sidequest variation so much that you have to do very difficult tasks in order to just get everything. It doesn't help that the game is so based around trial and error that you're basically given one chance, for a lot of the pieces, then you have to turn the clock back.