Use a mutant awakening gem on AA or ICEMAN
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I completed 5.3.6 and not only did a I pull a 5 star rank up (mutant) I pulled a 5 star mutant awakening gem. Who benefits more AA or Iceman, or the 3rd option wait for someone else ?
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EDIT - I forgot to also mention that there is a biohazard path in every part of 5.4. If you haven't 100% that yet, Iceman is the answer here.
That being said, new mutants are awesome too. Sabertooth, Domino, and now Omega Red and Emma Frost. Your call, but Ice is a great option
Ici boi imo is one of the best option to invest an 5* AG in. his Sig benefits him a lot (he's still god or demi god without it). But honestly I would wait for Rogue. maybe even Emma if she turns to be a god tier champ.