I am interested in the structures of game engines and I am currently attempting to design my own in-house engine for an upcoming project. However, I get in a position where I get completely confused. This is mostly because I don't have enough experience or knowledge of popular game engine systems. Can anyone give me some advice on what the best game engine structure is? I realize that I will be getting everyone's personal opinion, but I am simply looking for ideas and explanations. What I mean by structure, by the way, is the different systems in a game engine. For example: state manager, entity system, scripting, scene management, message handling, etc. I'm having a tough time explaining this to my satisfaction, but hopefully you understand what I am getting at. Thanks!

Every engine out there has a different architecture, it depends on your requirements and the scope of the project.

Nobody can tell you that the know what the REAL or BEST architecture is, since there isn't one.

My personal suggestion would be to take a look at existing game and graphics engines and learn from the architectures.

Some suggestions:
Torque
Irrlicht
Panda3D
C4
CrystalSpace
OGRE
Dave Eberly's game engine (think it is at geometrictools.com)
Fly3D (if you can find it)

Thanks for the suggestions. I realize that there is no definitive way to design a game engine. I'm just looking for opinions and explanations on what you all think is the best structure.