I am no expert in Motionbuilder, I only learned enough of it to use it for retargeting. There are tons of free motions out there and it seems such a waste not be able to use them for Daz/Poser figures. MB is really good at that sort of thing. 

Actually I am not certain of what the best tool is for key frame animation, C4D or MB. But I know that nitty-gritty key frame animation in Daz or Poser is a total waste of time. But C4D or MB? I don't know because I don't know any of them enough. Creating animation is not really my thing I only want to use what exists out there.

What Daz is good at is building fast animations with aniblocks and the Puppeteer. It also can use motions from Poser for example the walk designer. But again, it has to be cleaned up the seams between the blocks is almost never perfect and the feet are dragging on the floor, that sort of things.