Well, in terms of helpfulness, Navi was definitely the more helpful fairy, but I didn't enjoy Navi. Tatl had a lot more personality than Navi and was a lot more enjoyable to have in the game, whereas Navi was helpful in the first playthrough but then just got annoying, and was never really a good thing in the game, aside from that one time helpfulness, of course. They could have improved on Navi a lot to make her my favorite, but oh well.