I'm not going to plead my case on which song(s). Pointless, to many opinions. But what I will say is that at least we don't have to sift through the years of 1990 onward because there has been nothing that could come close to being considered best rock song of all times. Has there been some good tunes songs, sure.. Sure there have been.
I will give a shout out to Nirvana's Smells Like Teen Spirit as a post 90's song. It really did open up a new sound in rock. Not that I personally liked the change, but it was very influential.