Zora Mask easily.
Upsides and Downsides to the masks: Deku: Ive got nothin. The flying was hard to control well (at least for me), and all it could do is shoot bubbles. I guess spinning was kinda fun.
Goron: Spikey Goron Roll!!!!!! (Where do those spikes come from, anyway??) But fighting with it was a nightmare.
Zora: Fast in water, easily controllable, electric (INSTANT DEATH, MUAHAHAHAHAA), ok-ish fighting on land. However, the fast movement in water made it hard to be accurate if you were close to something (Just one room in the Great Bay Temple that I repeatedly couldn't get through the opening because I didn't have enough space).
I can't say anything about the Fierce Deity Mask because I haven't gotten it yet. Just 2 more masks . . . .