What about Green Arrow. Not only is he kind of Batman isn't he sort off the Superman as well of the Arrowverse? He is were his entire franchise basically got started. Heck we even use his name in how will describe these shows by calling it the ARROWverse.
Well, technically, but it was
The Flash that transformed the universe from a grounded, street-level vigilante drama into the full-fledged comic-book universe it became. It's through
The Flash that we got crossovers, superpowers (other than Mirakuru), colorful costumes, time travel, the Multiverse, etc. So the Flash is as much a foundational figure in the universe as the Green Arrow is.
So, yeah, technically Oliver beat Barry to the screen by a year or two, just as Superman beat Batman to the page by a year. But functionally, in both cases, the two characters are collectively the founders of their respective superhero communities; they're the oldest veterans, they're the closest duo, and one has powers and the other doesn't. So which one came first is an academic distinction. For all intents and purposes, Green Arrow is the Batman of the Arrowverse, Flash is the Superman, and Supergirl is the Wonder Woman, and yes, that looks really clumsy typed out, but you know what I mean.
Besides, what we're talking about here is an earlier version of a character from an older continuity that's been folded into the current continuity as an alternate Earth, which is what the Golden Age Superman was for the pre-Crisis DC Universe. As of "Elseworlds," we can now say that the 1990
The Flash is essentially the oldest Arrowverse series.