I use the Huffyuv codec for rendering and editing, because it has a lossless compression scheme. I can compress, edit, and recompress the video without suffering any loss of image quality. It doesn't compress well (although it's better than full-frame no-compression), so I usually recompress to DivX or convert to MPEG for the final video.

Make sure you configure the codec to "Always suggest RGB format for output", because Huffyuv defaults to a YUV colourspace, and Poser uses RGB. You can enable alpha-channel support from the configuration panel, also, if necessary.