Good one!

By pop metal I mean a heavy metal song with pop music sensibilities, by "greatest" I mean a phenomenal display of song structure, lyrical content, instrumental and vocal prowess, a song that transcends its genre and era.

The KISS song is a great contender. It definitely transcends its time and genre, and definitely belongs on any Top 10 list, but I don't think it's all that brilliantly composed. Lyrically and structurally it's awfully simplistic, like a nursery rhyme. The solo is pretty uninspired too. But its iconic status is beyond dispute, probably because you can sing along halfway through the first time hearing it.