I really like the Zora Mask, with the fast swimming, dolphin jumping, finmerangs, electrical barrier, and guitar. If it wasn't for the nerve-racking Goron rolling sections of the game, I would've chosen the Goron Mask.
Who else thinks the Giant Mask should have turned us into one of the four giants rather than just a bigger version of Link? Walking around as a lanky legged moustache ball would have been excellent.
Goron mask was best for me because it made getting around easier, though I love the other two. Water skipping is probably the most useful ability to me because I hate swimming in games and Zora was great because playing the guitar was super fun.