I never really put on the Postman's hat, the mask of scents, and the stones mask. I mean I've put them on atleast a few times just to see what they look like, but I don't put them on as much as I probably do other masks in the game.
I don't particularly use the Don Gero's Mask, the Mask of Truth or the Postman's Cap other than associated sidequests for the Postman's Cap. Don Gero's Mask is just dumb.
I would have to say the Great Fairy mask. I always use random masks, but I never seem to use this one. Last time I played I did try to use it more, but its kind of a hassle.
Probably the Circus Leader's Mask in the N64 version.
The Circus Leader's Mask actually does have a use in the original game: making a certain sidequest extremely easy. However, by the time you've obtained the Circus Leader's Mask, you've already done that sidequest. You could try the sidequest again and witness an Easter egg or get some rupees, but it's mainly for fun at that point. At least you can use it to get a 7th bottle in the 3DS version, or so I hear.
I'd have to say the Mask of Scents. I think I only used this mask once just to see what it did and to see how the Swordsmaster responded to Link wearing it, but other than that, its a mask I mostly ignore except to just get it to obtain the Fierce Deity's Mask later.