It really depends on what the rest of your roster is like. E.g., Stark is a great option against the CapIW boss, but if you already have Ghost/Hood, then you probably don't need Stark in that fight.
Mags is obviously great against the robot bosses, but if you already have Medusa, then you don't need him as much (though Mags also covers metal bosses).
Shang Chi is a great option against the Champion boss, but if you already have someone like Doom, then you might not need him there as much.
In a vacuum, I think Mags, AA, NF, or Shang Chi are the best champs. Stark gets consideration b/c he's a 6 star.
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Mags is obviously great against the robot bosses, but if you already have Medusa, then you don't need him as much (though Mags also covers metal bosses).
Shang Chi is a great option against the Champion boss, but if you already have someone like Doom, then you might not need him there as much.
In a vacuum, I think Mags, AA, NF, or Shang Chi are the best champs. Stark gets consideration b/c he's a 6 star.