I'd say the Circus Leader's Mask from MM was the least useful. Its purpose is to make the Gorman Brothers stay away from you during the horse and cart chase, which gets you the Romani's Mask. HOWEVER to get the Circus Leader's Mask, you need to have the Romani's Mask first anyways to gain entry into the milk bar. By that time in the game, the Circus Leader's Mask's purpose was moot.
The horsecall was also quite useless. By the time that you get it in TP, you already had the ability to warp so using Epona would waste time anyways. Unless you just want to ride her for fun, the horsecall gets little to no use. The telescope from WW was also pretty useless, as well as all the masks from OoT (if they count).