Go All Out has the potential to be the highest damage attack in the game, and its in leadership of all aspects!
The energy resource requirement is fine on a 2-cost card, but exhausting your hero as a cost means we need to compare the overall damage of this card versus the opportunity cost of missing a basic attack power. Generally the lower a hero's ATK and the higher their THW and DEF, the better this card will be for them.
Damage can obviously be increased using stat boosts. (Remember that ATK boosts would also increase a basic attack.)
3 Damage Base: Ms. Marvel, Quicksilver
4 Damage Base: Adam Warlock, Drax, Hawkeye, Iron Man, Nova, Rocket Raccoon, Spider-Ham, Valkyrie, Wasp (tiny)
5 Damage Base: Captain Marvel, Colossus, Doctor Strange, Spectrum (energy form included), Star-Lord, Thor, War Machine
6 Damage Base: Ant-Man (tiny), Black Panther, Black Widow, Captain America, Gamora, Ghost-Spider, Groot, Hulk, Ironheart (version 1), Nebula, Shadowcat, She-Hulk, SP//dr Suit, Spider-Man (Miles Morales), Spider-Man (Peter Parker), Spider-Woman (including Superhuman Agility for playing the leadership card), Venom, Vision (Dense)
7 Damage Base: Ant-Man (giant), Ironheart (version 2), Wasp (giant)
8 Damage Base: Ironheart (version 3)
Averages 4 damage for 2 resources when taking into account missing a basic attack. This is not terribly impressive on the surface but it does scale exceedingly well with supporting cards.
Spider-Woman has the highest non-infinite damage potential, currently her magical Christmas land damage number can reach 58 damage, or 64 with Doctor Strange helping. The actual number would be a lot less but it is still possible to one-shot a villain stage without gimping the entire deck.
Ant-Man, Spectrum, and Wasp are all even better than they seem because they can very easily trigger Moxie.
Quicksilver was already using leadership for OTK combo plays and this leans even further into that.
Yet another reason Doctor Strange loves the leadership aspect, as it gives him high burst damage with his Magical Enhancements that he previously was not really using.
Finally, steer clear when using Venom because the opportunity cost needs to include not using his two Venom's Pistol on his basic attack. It is also exceptionally poor for Ms. Marvel who needs to give up her Morphogenetics.