For me, it's any of her early stuff.

KOREAN
[1] ID; Peace B
[1.5] Don't start now / Jumping into the World
[2] No. 1
[2.5] Miracle

JAPANESE
[1] LISTEN TO MY HEART
[2] VALENTI

All of her albums, both Japanese and Korean, have some excellent tracks. I just find her earlier work much more consistent and cohesive.