I'm pretty much with Zeke on this. Ayu's ballads for the last few years have been "acceptable" rather than "fantastic" and honestly I can't differentiate between them in my mind for the most part.
"Zutto" sounds like Pray part II to me and Pray is very... well, it's too deliberately "epic" and it sounded watered-down to me, and this is no exception. She's done too many of this sort of sad-slow-dance ballad, with an old-fashioned melody, and the same sort of arrangement as a gazillion other songs. "petal" improved on this style of song as much as possible by making it a bit more intense, but even that one didn't have enough oomph.
"Last minute" is the closest to an ayu song that really recreates her previous sound, but it's for two reasons - one of which is good, one is bad, and they're both really obvious... The good reason is that it has a VERY dynamic arrangement, which I miss terribly! It's very simple at first and then kicks up to a ton more instrumentation really quickly, which is something she used to do often. It's great for creating a bigger impact. The BAD thing is that AYU HAS USED THIS CHORD PROGRESSION LIKE SEVEN THOUSAND TIMES. It's a good one, but once you've heard A Song for XX, I am..., SEASONS, Life, et cetera, it does lose its impact after awhile. The mix does sound a bit flat though, which upsets me because it uses mostly real instruments rather than having a synth based arrangement. Overall though, it sounds like the type of song fans expected RAINBOW to have on it but it didn't, so it's probably my favorite of the three. I prefer dark songs anyway, I feel more "at home" with ayu when she does songs like this.
As for Walk...my issue with this song is a weird one - I feel like the melody of the chorus & verses should be switched. They're TOTALLY BACKWARDS! I like the melodies very much, but the chorus sounds more level (aside from that one high note), and the verse sounds way more catchy & memorable and also has that "hanging" final measure, you know how J-pop songs back in the day used to do that thing where the last line of the chorus would be just one measure longer than it would be in a western pop song? I always loved that, it added drama in this very cool subtle way, and it was very very uniquely Japanese to do it... but here, it's the verse that does that! GAAHH. It's backwards. This song is backwards. lol