I personally like TerraLand and or WorldComposer. It generates massive terrains within minutes, as long as you aren’t walking on the terrain and just flying above it they look really amazing also.
Good luck
TerraLand:
Even though it has 3 stars I still consider TerraLand as the better option.
It just has more of a user friendly feel and has twice the customization when it comes to detailing the map. WorldComposer is nice though and is less expensive x)