Roxy Music - Love is the drug (best) More Than This (iconic)
side note: Roxy Music never had a big hit in the US (More Than This reached, like 100th on the charts here), but they were so influential that they got voted into the Rock and Roll Hall of Fame anyway.
I got Fairies Wear Boots for Sabbath and Flying High Again for Ozzy. But both bands are really impossible as there are too many songs in the same class, like a dozen for both.
I think if the title was “most iconic” instead of “one clear best song” this would be easier. Like definitely Freebird for Skynard, Stairway to Heaven for Zep, Smoke on the Water for Purple, but in no cases are those songs those bands best.
That’s a tough one. Obviously “Genie” was what put Christina on the map, but is it culturally recognized as her best song? I’d argue “Beautiful” did far more to propel her from one-hit wonder status to enduring pop phenom.