So, it's not quite as cut-and-dry for Marvel's First Superhero as it is for DC's.
Supes gets all the credit at DC because he's on the cover and is the lead story for Action Comics #1, despite Zatara the magician (Zatanna's dad) also debuting in the issue. Plus, Clark's first tale had actually already been crafted 4 years earlier before finally finding a home in AC1.
Similarly, Original Human Torch gets most of the credit at Marvel because he's on the cover and is the lead story, despite Sub-Mariner making his debut in the same issue.
BUT ... Namor's tale actually debuted months earlier than MC1 in the unpublished Motion Picture Funnies Weekly #1. Weirdly, Marvel officially counts that unpublished story as Namor's debut.
Basically, if MC1 is the 1st Marvel issue, then MPFW1 is the 0th Marvel issue.
Really, Submariner should be the historical backbone of the entire MU, as he stands in such direct contrast to Superman (the historical backbone of the entire DCU).
"What I sought, I could not obtain."
"This is a meaningless battle. We are two madmen engaging in senseless folley."
"I will kill, I will let live..."
--Genuine Fake Priest Kotomine Kirei
True enough. The characters before Superman faded while Superman gets credit as the first though he is really the first that is still around to a significant degree. One could say Namor gets credit over the Human Torch because of his success though if we really get technical, as Marvel Comics by name, the FF are considered the first.
Power with Girl is better.