gm_atomic
This map is very beautiful, making excellent use of a 3D skybox to create an enormous looking landscape. There are several buildings and a large networks of tunnels to explore. There's space for building without it feeling tacked on, and the map is also useful for machinima and comics.
gm_bigcity
Made by the same person as gm_atomic, this map is a lot less pretty, but it has to be considering the sheer scale. This pretty much pushes Source to the max. It's enormous, and a lot of fun to build or race in. It makes good use of levels, with sewers, a raised highway and the city streets. Another map useful for machinima and comics.
gm_construct_flatgrass
In my opinion the ultimate construct map. It has space to build planes, trains, boats and cars, and much more. I know garrysmod.org is flooded with construct and flatgrass remakes, but this one really is worth downloading.
