Impossible to answer that question. Everyone might have an opinion, some Roman dude in the year 120 AD might give a very subjective answer as a guy in the year 2212, who might quote me and name the local gig on Mars.
I live now (who would not claim that) and I would name a band my father could relate to more than I can, though does that matter? Best song ever? This question cannot be answered, the best song ever does not exist and would so much depend upon the moment and situation and person…
My personal entry would be ‘Love reign O’er Me’ by the Who (Can you see the real me? Can you? Can you?…)