Mecanim - Unity 4
Although Unity has some issues, and it's a little lacking in the way it can handle particles and lighting in comparison to some of the other engines, it can allow for a lot of stuff to be plugged in at least. But to make up for some of those short comings, Unity 4 looks to be doing some very cool stuff.
If you are interested in making games, I'd 100% suggest watching the video here:
This shows of the new Mecanim animation tools that will be built into Unity 4, and it's pretty damn awesome! It's simple, it looks like it will come with a library and it's re-targeting is a super time saver.
Mecanim still needs some work, and there are some bits and pieces missing, but its a really good starting point for something that could have a huge change on the way we build games, and how they look when in motion.
With this coming, plus the good relationship and support they have with Nvidia, Unity 4 is looking like it could be a good option going forward. But the super huge downside is that the cost appears to have gone up. Unity was previously a decent entry level 3D engine, but now it's a $3000-$4000 package if you want to use it for actually making and releasing stuff.