All time is pretty hard. I guess that would be the songs I still fall in love again from time to time even after playing them to death.
Ayu Fairyland, Sunrise ~ Love is all
Namie Amuro Wild
AKB48 Beginner
... to be honest, most of my fav. all time songs are outside J-Pop right now. But I'm sure there'll be a time when Japan will have a more diverse mainstream music again and then I'll be able to think of more songs

.