Once you get to the last month of the game, the themes for the locations around the city get changed. And the city theme, Memories of the City is the best one of them all. The more somber-sounding theme really fits the darker look of the city, as well as the sheer weight of what the characters are preparing to do.
Living with Determination often plays during sad moments and when someone reaches the resolution they need to evolve their persona. A remix plays after New Year's Eve in the dorm. It pumps the player up to prepare for the arrival of Nyx.
School would be much more popular if they piped the upbeat and catchy Want to be Close into the halls. Oddly, take away the percussion and piano and you've got the appropriately titled Tranquility.
Mistic (sic), which is the basis for the themes of the different Tartarus floors and plays during one of the tensest moments in the game (when the player decides whether or not to kill Ryoji). It's based around a deep relaxing piano and string melody, but avoids Soundtrack Dissonance.
When The Moon's Reaching Out Stars, an incredibly catchy and upbeat town theme. To contrast with this song's fast-paced, almost techno feel, Persona 3 Portable adds the equally catchy and upbeat Way of Life for the female protagonist.
Sun, one of the new overworld themes added for the female protagonist in Portable. Whether you think it's So Bad, It's Good or just plain awesome, one thing's for sure: it'll never get out of your head.
The Spring of Birth movie brings us "More Than One Heart". Appropriate to the movie's title, the song makes you feel like you're out in the middle of a spring day, basking in the sunlight.
Fate is in Our Hands, the theme for the second movie, Midsummer Knight's Dream. Lotus Juice's flow is absolutely jaw-dropping, especially the section from around 3:20 to 3:50.