If the criterion is that it hasn't been overplayed then I change my nomination to:
The Godfathers - Birth School Work Death. Seriously, check this absolute beauty from the 80s out. Absolutely effing superb.
https://www.youtube.com/watch?v=QO5dcW0P75M
Seeing as this thread hasn't reached page 2 yet and we've already had Bohemian Rhapsody, Sweet Child O' Mine, Back in Black, Freebird and Kashmir I dare say that there's a very good chance that ANY candidate for greatest rock song ever is likely to have been over-played, over-covered and more...
Was going to go for Deep Purple 'Hush' but I think this one is my candidate. Ram Jam Band - 'Black Betty'. Love, love, love this.
https://www.youtube.com/watch?v=ZbjyuDYtAtk