I'd like to know what are the best open 3D game engines.
The OGRE engine has always impressed me. It has been around for a long time, it's widely used and totally free with an awesome community to boot. This year they switched over the the MIT license. (It used to be LGPL).
It isn't a game engine per-se. It's really an opinionated, well structured graphics engine that is well suited to games. You may want to throw in OpenAL and Bullet Physics, too. There are lots of tools built around Ogre for your art pipeline, but expect to do a bit of work fitting the pieces together.
One caveat: OGRE's object oriented bit is all C++. So, it isn't wonderfully easy to get OGRE bindings for any language out there, but there are various wrappers available.