Here are characters I feel confident about using in CaVs so far. I plan to update this list if and/or when I get better at using other characters. It's roughly in order of how comfortable I'd feel using them.
Composite Earth-616, with or without prep, pretty much any scenario.
Original/post-Superior/composite, with or without Parkertech Spider-Armor, with or without prep.
616 T'Challa, with or without gear. I could probably do prep battles with him too, but I'm not 100% confident about that as of yet.
616 Hercules, both fully powered and depowered.
Classic incarnation, dying incarnation, and spider-powers incarnation, with or without prep.
616 Thor, both Worthy and Unworthy incarnations.
MCU Quake, both depowered and with powers. No prep battles.
616 Tony Stark, composite armors, with or without prep. I can probably do armor-specific matches as well for Extremis, Bleeding Edge, Superior, and Model Prime Iron Man.
Prep battles only.
Pre/post Space Knight amp for Venom, Anti-Venom as well.
Post Spider-Island resurrection.
Both the Pat Mulligan or Eddie Brock versions.
CW Green Arrow, hand to hand/melee battles only.
616 (Venom/Anti-Venom/Toxin).
616 Kasady version.
Osborn, can do both with prep or without prep.
