Leadwerks Game Engine is the easiest way to build 3D games. With a rapid development pipeline, support for Lua and C++ programming, and plenty of learning materials, Leadwerks is the perfect way to learn to make games.
Leadwerks 3 has a lot of improvements over the earlier versions. The editor's interface has been vastly improved, and it seemingly resembles Valve's Hammer Editor a lot now. This also means that the engine supports brushes, which is ideal for laying out interior level designs, and the workflow is really smooth, with textures and model importing being easy as pie. Community service is also very good, and Leadwerk's creator Josh frequently answers questions on the website's forum.
Be warned though that this engine is still mostly geared towards programmers, which means you need to have an experienced programmer on your team to do the compiling or learn it yourself.