Trollface. Although heavily meme based, the trollface has emerged as one of the more important, and solid standing methods of online visual communication over the last few years.
Not a logo per se, but it’s one of the figurehead symbols of natural/organic user generated content. Not the rubbish that agencies want people to make for their brands.
I like it because the expression is spot on, it doesn’t ever really lose it’s power like other more time-specific memes and plus I’m always a sucker for ms-paint art-rape (probably some sort of sub-conscious rebellion against vector stock.)