This was Aoi Yuuki's best role.
I'm trying to think of a better one but I'm having a hard time of it. Tohru really was amazing.
Ringo-chan in High School Boys is the only one I can think of off-hand that comes close, but it wasn't really a great role just because of Aoi Yuuki.
How wrong you are.
But all of Aoi Yuuki roles are her best role.
Kaname Madoka (mostly for Episode 10 and 12)
Hotblooded Aoi Best Aoi.
But really she's great no matter what because she's literally 2.5D perfection.