It's definitely "The Avatar State"... It's the first and only episode (apart from the finale) where we got to see some past Avatar's. We got to learn about The Avatar State, something that was crucial in that season's finale, and see the ultimate height's of bending arts. Like for Waterbending, it was creating and controling a massive tsunami, for Earthbending it was bending two very, very large rocks almost with ease, for Firebending, it was erupting a volcano, and manipulating Lava/Magma, for Airbending it was blowing down a forest with heavy winds... It was AWESOME! =D
The Boy In The Iceberg: 0
The Avatar State: 3
Awakening: 0