There were a couple of useless masks that I found actually quite a bit useful, even if they were for one quest. The Stone Mask, for starters...I wonder why that's an option, because that mask really was useful, especially in scenes of ambush. Granted, it wouldn't work on bosses, mini-bosses, or certain enemies, but it definitely helped outside of those places.
Another favorite of mine would have to be the Bremen Mask. Now, I like it because, being the marching band nerd I am, I like how Link is actually in step with the song (if you do the timing with the walking right), and he gets an army of farm animals to go with him! Now why it makes baby Cuccos mature (I haven't a clue what to call them), I haven't a clue. It could just be the magic of the ocarina or up-beat marching music.
The Kamaro mask was, well, strange. I sort of question why it requires the spirit of a mask to get Link to do a dance, but I found that dance sort of odd. The Mask of Truth was also decent, but unlike Ocarina of Time, it was more about the stones telling you about how to obtain masks than actually getting secrets of the people of Termina.
Stone mask, It's saved me on many occasions! It's probably one of the most useful masks in the games! Most enemies can't see you and won't attack you! Very helpful in Ikana for those annoying Real Bombchu's (Charing Rats with bombs) And those pesky birds!
I chose the Garo's Mask. It doesn't annoy me or keeps me from attacking and it looks very cool. ^^
And I also chose the Blast Mask. Whenever you play the Song Of Time, you lose your bombs and if you find a destructable wall then, this mask saves you a lot of time an rupees. So I don't think it's really useless (and you can use your shield to prevent damage. xD).
I'd say that my favorite useless mask from MM would be the Keaton Mask. I loved running around Termina with the mask on! It made me feel like I was kinda like Kafei in that I was hiding my identity as I was going around helping people and defeating the dungeons. I also found it kinda funny whenever I wore it during the Kafei and Anju sidequest.