"Herp Derp" is a good extension.

Here's a visual which I shamelessly stole from the Herp Derp website:

The extension is useful, obviously, because YouTube comments are just all-around terrible, but more importantly, it prevents spoilers, like if you want to watch all those Game of Thrones info things (example) for the TV show, without worrying about getting spoiled by jerk YouTube commenters who somehow managed make it through one whole book (or read the summary on the books series wiki).