the best video will be the same res as the display (1024x768) DVD's are gonna be a little under (though if they're full quality, not iTunes downloads or internet downloads/streaming they should still look good) at 720x480 and 720p is just a bit bigger at 1280x720 (bigger is better than smaller)
anything smaller than 1024x768 isn't gonna look as good stretched to the ipad screen.