I liked Tatl a lot more than Navi. For one, the 'implied' romance between Link and Tatl (more of a bond) was much more "developed" as seen in the ending of Majora's Mask versus Navi's supposed jealousy. I also found Tatl to be a lot less distracting, and despite her mean personality, she wasn't as ****** as some of the other helpers of the Zelda series. Another thing is, she didn't turn out to be completely useless when facing the final boss! Seriously, if you can lock onto him, I'm pretty sure you can find out his weakpoint or at least hint to what needs to be done! Don't give me some halfdone "durrhurridkwat2does" answer!
For those reasons and more, Tatl will always be better in my eyes than Navi.