1. World Building Bundle
World Building Bundle is a complete package for terrains and you will not need any other package for terrain generation
2. Simple IAP System
If you are planning to build a freemium game then In-App-Purchases will be a part of it. Simple IAP system makes this simple.
3. Super Texture Collection
While making 3D games you will often find yourself searching for textures. Super texture is a collection of 600+ textures of your game. It has 300 hand painted textures included in it.
4. Casual game SFX
For a new game developer getting sound effects for every situation can be difficult. This package from Unity is a huge collection of game effects used in casual gaming.
5. Spline mesh
Spline allows you to create curved meshed in Realtime and the best part is all the docs, source code and the asset is available for free.