I agree with all the posts so far saying Synchronoise and Quash are useless. Now some of my own choices:
Bide: Why is it useless you ask? If you have a bulky Pokemon that can comfortably take a bunch of hits, you can retaliate with double the power. Except that - you know - your opponent could use the Bide turns to set up, or simply switch to a hard counter, instead of hitting you with weak attacks. At least Counter and Mirror Coat have a surprise factor to them in that you can use them to take down Pokemon that are otherwise left unscratched by your STAB moves.
Rock Smash: It's so weak that even with STAB factored in it still doesn't even do much damage to Pokemon that are 4x weak to Fighting (it doesn't help that a lot of Fighting-type Pokemon don't learn that many Fighting moves through level-up). The Defence drop doesn't even have a 100% chance of happening. Pretty much every other HM in the games is at least usable in battle. Even Flash at least lowers your opponent's accuracy, allowing you to hax the AI to death. Maybe if they upped the BP to 60 or even 80, it would actually be useful for more than just breaking rocks.
Metronome: It largely depends on luck whether it is actually helpful or not, but with thousands of moves it can mimic, the odds of coming up with a good one don't look so good.