Most Overrated: Skyward Sword
This answer feels a little generic coming from me, but my opinion still stands. The concept of something being "overrated" means that it is getting more praise than it deserves. While there are some Zelda games I feel get less than they deserve, SS is the only one I feel gets more on a general basis. Even though I like SS as a game, I feel "meh" about it as a Zelda game and I just hate it as a series game. But I'm not ignorant (I think) so I payed attention to people whom actually loved the game. The reasons I was told why it was so great, I simply couldn't agree with. Just one of those times where I have to say "Well, I'm glad you enjoyed it more than I did." So while it may not be the most loved Zelda game, I feel that the reasons it is liked are still overrated.
Most Underrated: Twilight Princess
This one was a bit harder. It was a choice between OoT, TP and ST. In the end, I chose TP because I've heard more negative things against it than the other two. You guys have heard it from me before, I freaking love this game. Not only that, but I believe it's a great Zelda game (though not the best), a fantastic game in general, and a rediculously rupee series game. In the past I've described it as "Zelda all grown up" and I stick by that. Our little series Zelda was now fully grown and had lost it's innocent charm, however it was still Zelda deep down. Despite the complaints about it being different, I saw PLENTY (capitalizing things makes them true) of Zelda elements. The atmosphere may have varied from passed games, but it was still Zelda at its core.
Then there's the complaint about it being too similar to OoT, which again I defend with "it's a sequel in all aspects." Because we're used to sequels that are ridiculously different from their predecessors, it's more difficult for the Zelda universe to accept a legit sequel. It's really hard to call TP a clone of OoT without calling games like Jak 3, inFAMOUS 2, and Uncharted 2 clones of their respective predecessors.
And I understand that most Zelda players want a more difficult Zelda experience (me too) but the difficulty was done, successfully I might add, so outsiders would find Zelda more appealing. And it's not like we Zelda fans weren't compensated. Nintendo still made TP extremely innovative and fun for our pleasure. Imo, fun outweighs difficulty. After all, isn't the reason you want something to be more difficult is so that it can be more enjoyable?
Those are my thoughts. Have fun tearing them apart.