That seriously is not a hard question for me to answer; EXODUS, hands down.
I felt that EXODUS had a spark about it, something that drew me into the songs. Sure, I only listened to Kremlin Dusk at first, ignoring the other songs, but as I gradually listened to the album, I loved it.
I just feel that the album is full of "Utada-esque" quirks that make me love her. I think EXODUS and its themes, styles and song-writing perfectly encapsulate why I love Utada. Her uniqueness, her voice, her stylistic music.

I mean, don't get me wrong, This Is The One is a good album as well, I just feel that TITO is missing something... because it just feels different compared to EXODUS (well, duh, I worded that badly...)
I automatically liked every song on This Is The One, which is really weird because I usually only like two or three songs off an album usually (not just Utada's, but every album), but This Is The One is so easy to listen to.
And with EXODUS, I hated most of the songs to begin with, but they grew on me, which I think is why I love it so much - the album "made" me like it, haha.

Anyway, when it comes to it, EXODUS really does shine in its own light, and I really hated the fact that Hikaru called the album a "mistake", I mean, Kremlin Dusk, no doubt her best song to date is on there, and no song could compete with that kind of magical talent, ever!