Well, it's a bit like asking " Who is the beautiful women in the world?"

Why I use that example is because, back in the 60s, Raquel Welch carried that title but I didn't see it at all as because she used to remind me of my sister!
BTW, whenever I would mention that connection, back in the day, some bloke would always ask " Would you introduce to your sister?"

AS to the greatest pop song. Well, I think that you would have to have different categories as comparing e,g. a 50s pop/rock song with a Cole Porter, Gershwin, Kern type song would be like comparing chalk and cheese.