02-26-10: TvadI'd also say most covered song would qualify as best pop song. There has to be a reason why it's covered the most.
Whichever has sold the most records. That's what pop music is all about. Once the answer is found, no further debate is required.
I think for decades, that title was held by "Louie Louie" by Richard Berry circa '55/'57 but made infamous by The Kingsmen in ~1963. There have been something like 1,500 covers/versions of that tune recorded.