Gah...this is very difficult.
I guess if I had to choose just one, I'd pick "First Love" by Utada Hikaru. I'm not nearly as big of a Hikki fan as I once was 5 years ago, but it was one of my first jpop songs ever. I only had a crappy version at the time, but I listened to it over and over.
It's gorgeous and you never get tired of it, too.
