My most popular video by far is my "Let It Go" one from Frozen. I had uploaded a version of it around October or November and by the time January of this year rolled around, it had 15k views and it was one of my most popular videos at the time. It was only surpassed by my Star Trek: TNG clips. I then noticed that the quality of it had somehow gone from 480 to 240 and it looked terrible. So I made a 1080 version and put it up at the end of January and put an annotation on the old one directing people to watch the new one. Most people ignored it and my new one got almost no views from it, so I just deleted the old one.
Toward the end of March it had around 50k views, mainly from searches, but I still only had 6 subscribers (the most I had ever had by this point). But something amazing happened in April. A popular channel with more than half a million subs uploaded a highly anticipated video called "Let It Go - In Real Life" and because mine already had so many views, it became a Suggested Video on it. My views skyrocketed and I gained more than 600k views from April 9th to April 20th. Then it died down, but the initial fame made it much more likely to be a Suggested Video on other ones, not to mention driving traffic to a lot of my other videos. Right now it has 6.5 million views, with my 2nd place one having 433k.