There are several and you'll definitely get responses from people who have various favorites (plus the seldom-helpful reminder to read the sticky thread). Anyway, for the money, I like Caligari's GameSpace. The simple .X animations of my own models that I created using their tutorials worked in DBP the very first time and I've been able to break apart and animate 3rd-party models (like the WW II tanks from Geo-Metricks) that are not animated when you buy them.
There are some "canned" animations that come with GS that I've used unmodified in DBP, namely the animated "skeleton" that walks with a very natural gait. I first plunked it down onto a DBP matrix onto which I'd textured several city blocks (streets, sidewalks) and set him to walking around and it worked. Next I tried some really basic "limbs" attached to the skeleton -- OK, I took a cube and painted a smiley face on it and glued it to the head "bone" and some cylinders for arms and legs, exported it, and that worked also.
So I'm pretty confident that if I ever learn to model nice-looking human body parts, I've got a library of basic animations already waiting for me and I know they work in DBP. And at that, the limitations are with my skills, not GameSpace...
