Emerald Dimension
In the Emerald Dimension you can find the Emerald Biome. It has a really green grass colour and many trees and giant mushrooms.
What's a Emerald Dimension without emeralds? Nothing! So, you can find many more emeralds instead of searching many hours in an extreme hill biome...
You can also find Emerald Cows and Emerald Wolves there.
Tip: If you're playing with other mods (which add some dimensions too) you should change the dimension id from 2 to e.g. 20 in the config.