On almost every project I've ever done, voice first-- usually after the script is done, but before boards/animatic. It's not just about lip sync; the voice acting informs the animated acting, timing, mood etc. I did one big student project that was animated first, because there were no lines, so the voice was more like an accent or sound effect.
After the animation is done, then layout (bg drawings). You have to set the stage before your actors move around in it. THEN animation, rough and clean or however you're working. Once the animation is clean/colored and bg is painted, comp it all together, and combine it with your sound in the editing software of your choice.