Water:
With Avatar State: Aang rising the sea level
Without Avatar State: Korra freezing Kuvira's mech
Earth:
With Avatar State: Kyoshi splitting the peninsula to create Kyoshi island.
Without Avatar State: Kyoshi holding up the roof of the tea house when Jianzhu was trying to collapse it.
Air:
With Avatar State: Aang blowing a rock to smithereens
But, I would like to say their is a valid argument for this being the most powerful:
Without Avatar State: Roku's airball.
Fire:
*With Avatar State: Roku melts metal. I know this seems like such a little thing, but he was able to shoot a bunch of fire, melting only the chains (something firebenders are bad at) and taking out the wall.
**Without Avatar State: Roku summoning a giant Sozin's Comet level fire right here.
*The reason I didn't count Aang's fireballs was because he was also getting a boost from Sozin's Comet not just the Avatar State, thus not allowing a fair assessment of that feat
**The reason I didn't count Roku's fire in the cave was because similar to Aang, he was getting a boost