Noob needs advice on who to R4 next

Captain Marvel.

However, do what you like Rank the ones you like fighting with. Everyone has an opinion, but they don't have to play the game for you.

Theremon

Captain Marvel hits hard and is a good attacker. Mine is r5 sig 99 and I use her all the time on AQ map 5. Has synergy with Wolverine, too.

Magneto can take a big hit and survive, and has synergies with Wolverine & Magik, so that would be a good choice, too. I just brought mine to r5 (after Wolverine & X-23...I keep getting Mutant t4s twice as much as anything else).

My Vision AoU is r4 sig 99 and will probably be my first Tech r5, if I can ever get the 3rd t4. He's better duped, though.