YouTube Ratings Preview is a Firefox extension (also available for Chrome) which makes it easy to find the best-rated YouTube videos.

Once installed, a Likes/ Dislikes bar under every video thumbnail will give you an immediate idea of video popularity (just look for clips with lots of green, and minimal red).

If you're looking for something more precise, just hover your mouse cursor over the bar to see the percentage of likes and dislikes, and the number of votes.

And it's worth checking out the YouTube Ratings Preview value, too - it "rates every video with an intelligent score from 0 to 10".