The Beyond Corporation
In contrast with Oscorp's squeaky-clean professional appearance, the Beyond Corporation is wall-to-the-wall comic book lunacy. This place has everything: Elvis MODOKs, Baby Iron Men, Kree pirates, Cyclops brontosauruses, snakes piloting biplanes, and a giant ape dressed up like Wolverine.
The stage is basically a grimy corridor with whatever absurd experiments that the Beyond Corporation's cooked up going on a rampage through it. The double-page spread pictured above is just a sample of the madness that this stage could bring (it's from issue 11 of Nextwave: Agents of HATE, an issue that mostly consists of double-page spreads showing absolutely ridiculous fight scenes)
Plus, it's not like there's one obvious pick as far as characters go -
Monica Rambeau is likely the most well-known, between her MCU appearances and her status as an Avenger, but there's also
Elsa Bloodstone (a monster hunter who showed up in Marvel Ultimate Alliance 3),
Machine Man (a robot that's like a living Swiss army knife who first appeared in an adaptation of
2001: A Space Odyssey, of all places),
Boom-Boom (a member of the X-Men who can make explosions), and
The Captain (who used to be called Captain ****, until Captain America beat him up and washed his mouth out with soap).
Basically, Nextwave: Agents of HATE is an amazing series, it's a shame that it only lasted twelve issues, and I wanted to nominate something from it at least once.