It should only be the highest viewed video of the song, whether its official or not.
Some artists don't even have an official channel, or that the official video is released much later..so I'm okay with it.
EDIT: OMG I read it wrong!!! What?!! Every single video on YouTube with the song on it, whether ots official or not?? So covers, parodies, even people dancing to it counts??? Wtf??
I guess that makes sense tho, the song's success can be measure by how much exposure and how many people had listened to it or know of it.. You come to 'know' of the song when you hear of it.