Do My Best ranks dead last among her songs in my book for one. I've never thought about why exactly it is offensive to my sense of aesthetics, but let's see if I can't put my finger on it.
First of all, the melody is much too repetitive. It goes on but goes nowhere. Second, I don't feel it works with the lyrics. While the very positiveness of the song that you like already works against my opinion thereof, I believe the aesthetic could work with a more subdued, or even just a better structured melody. As it is, the whole thing is too overpowering, hitting you over the head with how wonderful life is and not stopping. And I mean that in a bad way.
Ultimately, I suppose I find it lacking in the maturity I prefer from songs in general and Nakazawa's in particular.
Or the short version: I don't like the music or the lyrics.
I hope I was able to shed some light on my perception of the song at least.