I was thinking a Beatles tune but there is so many big hits that it's hard to narrow down one.
Let it Be?
Probably. I was doing the same, trying to think of the perfect Beatles song. I think it's got to be up there.
One that I do not think has been mentioned that has to be up there is:
Piano Man
I don't think I even know the song you are talking about?
I would definitely go with Bohemian Rhapsody, it was the first song that jumped to my mind. I think it's also stayed relevant through generations due to films.
For those who remember it from the time it was released they remember how popular it was. I was born in 83 so for me my first exposure to it was the iconic scene from Wayne's World. Everyone of my generation at least knows that scene, even if they never watched the whole film. I personally watched that film loads when I was a kid. For the younger generation it was recently in Suicide Squad, a lot of kids found a love for it due to that.
Another great thing about it is the fact that if you hear it you can't help but sing it (maybe just my opinion). It's infectious and singing it with your friends (in a car if possible), or in a big crowd is just the greatest. I could understand some people being sick of it but I think the vast majority of people haven't heard it enough to be sick of it.
The final thing in all of this is purely about how popular certain songs are in certain parts of the world, that just makes the biggest difference.