Biggest impact on the stream?
Scar & Sloth. [Tie]
Scar's attempts to create a truly free, fake political stream didn't work, but got us where we are today. (Mudslinging, freedom of speech, constant debates about censorship, etcetera.)
Sloth's war against the stream was unfortunate, but taught many of us an important lesson about hate speech. Last year at this time, we'd still be debating whether nazi speech was good or not. Now we can at least argue over what hate speech is as opposed to whether or not hate speech is good.