Err... you're kind of asking for the whole package, you want power, ease of use (for beginners), and you want it to be flexible. I think you can only pick two according to the cosmic laws
If you want something for beginners, I suggest using the blender game engine as a starting point.
It has:
ease of use and is flexible, however it is not so powerful when coming to making a commercial/semi-commercial quality game, far from it in fact. However it will introduce you to the basics of game design, it uses python, however since blender is open source I'm sure you can modify it (written in C I believe)
Another option is as mentioned DarkBasic, it has power AND flexibility, however, it is not suitable for COMPLETE beginners to just jump in and expect to have a full game in a few weeks. Maybe a small mini-demo, yes, but I assume you want something to help you make a game as you mentioned in that other thread.
Another option: Unity3d, I am not so familiar with this, so maybe someone else could clarify what unity offers?