Best way of optimizing 3D meshes?

What is the best way of optimizing 3D meshes? I generated a 3D model of a city using Autodesk Infraworks 360 but the 3D model of the city has very high number of polygons. I tried to use the Optimize modifier in Autodesk 3DS MAX but the results are not good. The 3D model of the city has a LOT of 3D meshes - water, ground, streets, buildings, etc.

A lot of them may b e identical, like replicated Buildings.
Try to modularize them. This way you only need to import one of each.

Elbow grease. Retopo the mesh. As stated above. Some buildings can be re-used.
Also it depends on your use case.

If its only seen from far then you can save a lot of effort. If its close viewing then you might have to rebuild every building.