I guess many people don't like Ai Otsuka because they can't tolerate her high-pitched voice. But I really enjoy her happy songs, makes my day sunnier. xD SMILY! Her crazy songs are awesome too.

But Ai is at her best when she does ballads. You gotta love her ballads. =3 Her voice is soft, it brings out a certain bittersweet vibe.

Favorite songs:
5:09a.m.
CHU-LIP
Kumuriuta
Pocket
Renai Shashin -Haru ver.-
Sakuranbo
Momo no Hanabira
Ame no Naka no Merodii
Kingyo Hanabi
Kuroge Wagyuu Joshio Tanyaki 735 Yen
Friends
Biidama
Smily
Planetarium
Ramen 3 Pun Kukkingu [Cooking 3 Minute Ramen]
Neko Ni Fuusen
Tears
Yumekui
Tokyo Midnight
Hoshi no Tango
Mackerel?s canned food
Yumekui
Katorisenkou
Kimi ni Kaeru
Natsuzora
Always Together

...I can't think of anymore. xD

On the Tokyo Friends Movie OST, it features orchestrated and piano versions of Friends and Yumekui. Tears is also on the soundtrack, which one of them is a guitar version and the other is orchestrated. I'm not a hundred percent sure, I just typed this from memory. xD Which isn't reliable...
Anyways, give Ai Otsuka a good listen! ^^