If not counting this thread, then 4chan's /co/ was the #1 thing I could think of 3 or 4 years ago. All of the worst lolcows, fandoms, manchildren, tumblrtards, spergs and neckbeards have been spawned by /co/, are connected by /co/ or behave exactly like your average /co/ poster, right down to behaving like children or wanting to fuck animated little girls who want to sit in the back seat of cars (like weebs but without the constant denial). Even the worst /b/ had to offer were usually anons who had been banned from /co/, like /b/ronies. Bronies were probably the pinnacle embodiment of everything wrong with /co/ and the internet as a whole, and they would eventually lead to the spawning of even worse fandoms and communities.
Nowadays there's just too many horrible things coming from every corner of the net (especially Twitter) to keep track of. So I guess the internet as a whole is the worst thing ever.