I also chose the Deku mask, because I wanted it to get some more points on the poll.^^
If it hadn't been for that reason, I really couldn't have chosen a mask. I like the Deku mask because I think that the deku's gameplay is just very fun (also, I noticed how you can totally destory Odolwa with the help of this mask recently), I like the Goron mask for the pure power of the gorons fists and for letting me roll over the fields around Termina, what is always so much fun. I like the zora for the same reason some others here like him as well: He makes water sections in Zelda not only endurable, but fun!
I also like the Firece Deity mask, though I thik it's not that usefull in any situation other the fight against Majora.