Unless you source mod the shiz out of it, or use one of the already available source mods....
Dark Basic is intensely easy to learn, as all the commands have straight-forward names like MAKE OBJECT SPHERE, or SET REFLECTION SHADING ON. There's also tonnes of tutorials that'll help you get where you want to go.....
FPS Creator's good for learning level design and lighting. It's good for a quick and easy game creator, but the visuals are very last gen, and the AI are pants-on-head-retarded. But there are some pretty good user made AIs, and there's a mod called EFX that gives it the more up-to-date features like Bloom, Fresnel Water and Motion Blur.....
Dark GDK is basically DarkBasic type commands in a C++ API. If you know C++, unlike me, you'll find this easy to learn....
The 3D Game Maker is designed for 5 year olds who can make their own simple little game in seconds. Sadly there's little or no ability to choose enemy positions, level design, etc, etc, so it's useless for any serious dev.....
Make your choice, and welcome...