I don't care
Well, I don't care. For me, Metallica and Rammstein are now more like "entry level" metal band for people that are figuring out their musical taste. So yeah - commercially speaking - Metallica is possibly the biggest one.And the biggest metal band is Metallica (sine 30 years!). Iron Maiden is on the same level as Ozzy Osbourne/Black Sabbath - sometimes less or more but way behind the Metallica and even Rammstein. These two bands' fan base is still growing with hosts of young fans - we can't say the same about Maiden, particulary in US.