First Love: First Love
Distance: Can You Keep A Secret?
Deep River: Deep River (Final Distance & Hikari tied for 2nd)
Exodus: Kremlin Dusk
ULTRA BLUE: Blue/Passion (tie)
Heart Station: Heart Station
This Is The One: This One (Crying Like A Child)

Her bests seem to be the title tracks LOL...

Today-

First Love: Automatic
Distance: Addicted to you
Deep River: Shiawase ni Narō
Exodus: Kremlin Dusk
Ultra Blue: Colors
Heart Station: Prisoner of Love
This is the One: Merry Christmas Mr. Lawrence - FYI

Not really a fan of Precious. Anyways...

First Love - Automatic (there's a special shine to this song which makes it very memorable and happy)

Distance - Kettobase! (friggin' amazing, she should perform this LIVE more often! The Hikaru No. 5 version of this song is utterly mind blowing!)

Deep River - Hikari (this song is magical in many ways, whenever I listen to this, it gives me mixed feelings, both happiness and sadness at the same time. And also, it feels as if I'm in another world, I can't describe how great this song is, even the "ahhhs" after the last chorus sound great! It's just so overwhelming.)

Exodus - Kremlin Dusk (well written lyrics that are easy to relate to, the way the song builds up is totally flawless. It's just very vehement.)

Ultra Blue - This is Love (the starting of the song sounds like VGM! It's catchy all the way!) and I'm actually had a hard time choosing between this and One Night Magic but ended up with This is Love. Btw why is there no music video for this? GAH.

Heart Station - Kiss & Cry (another catchy song and I love the pre-chorus even though they sound like Hotel Lobby but hey, Hotel Lobby is good so I don't mind how it's melody is used in this song as well! Just one complaint, why isn't she in the music video? Oh well. And the "natural high" part is lovely)

TITO - FYI (Utada changed a slow beautiful theme song into a great rnb track. Quite an accomplishment because she didn't butcher it, fortunately. Despite the song being so rnb, it has that eastern touch to it as well.)

Btw people, stop cheating! I know how incredibly difficult it is to select BEST songs from each album. Don't list 2 songs for one album, cheaters! :p

Precious - "How ya Doin'", if any...
First Love - "First Love" (nice love song. I can't believe she made this song when she was the same age as me)
Distance - Kettobase! (guitars! and very catchy chorus)
Deep River - Play Ball (I hate sports, but "throwing heart" sounds great) and Uso Mitaina I Love You (again, guitars and catchy chorus, lovely song)
Exodus - Kremlin Dusk and Animato. I love both, can't decide which is better
Ultra Blue - Kairo and Passion (Passion is one of her best songs, very original. Kairo also sounds like it was reversed or something, it's good when I want to rest)
Heart Station - Beautiful World (first Hikki's song I've ever heard) and Take 5 (a bit weird, bur beautiful, great lyrics, kinda reminds me Kairo)
This is the One - This One (Crying Like a Child)- lyrics written like a letter, and Merry Christmas Mr. Lawrence - FYI - I always smile when I hear this song.
Singles Collection vol. 2 - Show me Love (not a dream) - GUITARS! Hikki sounds just awesome, very strong and emotional song, it would be great if she recorded new album with some rock songs.

EDIT: sorry Ramenbox, it's impossible to choose just one, I've tried... BTW, so I'm not the only one who's noticed that Kiss&Cry and Hotel Lobby are similar

First Love - Movin' On Without You
Distance - Hayatochi (Remix)
Deep River - Sakura Drops, Travelling, Letters (Sorry but i can't choose out of them someone choose for me haha)
Exodus - You Make Me Want To Be A Man
Ultra Blue - One Night Magic
Heart Station - Heart Station, Stay Gold, Kiss&Cry (Can't choose again)
This Is The One - Merry Christmas Mr. Lawrence - FYI
Utada Hikaru Single Collection Vol. 2 - Show Me Love, Goodbye Happiness (So Good ^^)

wow i'm shocked that only two so far have mentioned Stay Gold.

Precious: "Close to you"
First Love: "First Love"
Distance: "Time Limit"
Deep River: This is tough i can't decide between "Letters" and "Final Distance"
Exodus: "Exodus '04"
Ultra Blue: I'll go with "Wings"
Heart Station: "Stay Gold" by far
This is the one: "FYI" the only one i really like in this album.
SCV 2: "Goodbye Happiness"

Here are mine:

Precious - How ya doin'
First Love - First Love
Distance - Time Limit
Deep River - Deep River
Exodus - Kremlin Dusk
Ultra Blue - Colors
Heart Station - Heart Station
This is the One - This One (Crying like a Child)
UHSC#2 - Show Me Love (Not a Dream)

First Love: Amai Wana~Paint it, Black~
Distance: Sunglasses
DEEP RIVER: Shiawase Ni Narou
EXODUS: YMWBAM
ULTRA BLUE: 日曜の朝
Heart Station: Heart Station
This Is The One: FYI
UHSC#2: 嵐の女神

Precious: Close To You (amazing cover, better than the original!)
First Love: uuuhm....Amai Wana or In My Room
Distance: Sunglass (sad song ?__?)
Deep River: Deep River!
Exodus: Kremlin Dusk
Ultra Blue: BLUE!!!!!!!!!!!!!!
HEART STATION: Stay Gold
This Is The One: This One (crying like a child)
SC2: Arashi no megami/show me love (not a dream)

First Love: Time Will Tell
Distance: Can You Keep A Secret?
Deep River: Deep River (greatest song in Jpop history)
Exodus: Kremlin Dusk (worst album ever)
Ultra Blue: BLUE
HEART STATION: Prisoner of Love
This Is The One: This One (crying like a child)
SC2: Goodbye Happiness