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 settings, too.
By default the ratings bar is only 4 pixels wide, but if that's an issue you can increase this up to 7.
And if you don't want to look at individual bars at all, then there's an option to highlight up to the best 8 videos on any page by drawing a blue box around them, so they're much easier to spot.