Plains for the look. Swamp for the gameplay.

Once you hit swamps it feels like the tutorial is finally over and the game really opens up. You can start building with stone and iron. You have a good reason to learn about meads. Enemies have diverse behaviours and can't all be rushed half-heartedly. Even the terrain itself can pose an entertaining challenge. The only real detractor I find is that it has the most disappointing boss battle in the game.