You can get the registered RiVa from one of the latest Amiga Future cover CDs (I cant remeber exactly which one, but you can easily use google for that).
Despite RiVA being the fastest MPEG player, as suggested, you can use FroggerNG, which is now free and has much more features than RiVA.
MooVid is only for AVI and quicktime/MOV video playback, but it is another, non related product.