Unity3dx features some of the best content packs, specifically optimized for the Unity 3d Engine. Buy one and download it right after!

Superterrain Vol 3: Warm Hill
Superterrain Vol 3: Warm Hill
Version: 1.0
The Superterrains are terrain mesh created to be low poly photo-realistic, suitable for PC/MAC but also for mobile platforms. Typically the smooth version takes 60k poly and the version for mobile around 3k only, with little to no difference even in close up view.
Price: $35
Superterrain Vol 1: Blue Ice
Superterrain Vol 1: Blue Ice
Version: 1.0
The Superterrains are terrain mesh created to be low poly photo-realistic, suitable for PC/MAC but also for mobile platforms. Typically the smooth version takes 60k poly and the version for mobile around 3k only, with little to no difference even in close up view.
Price: $35