You answered your question. Prof X is the obvious answer. Together with magneto, you have a mental barrier which is an extra shield. make prof x win 3 more fights and he gets a max of 4 cerebro charges. 1 cerebro charge is 25% ability strength given to the other mutants battling with him. 25 times 4 is 100%. So he's basically giving his mutant or x men tagged team 100% extra power when they use their special attacks.
Professor X is a good synergy champ but he's also a good champion on his own if you can play him well. The rampup might be slow at first but the payoff is really satisfying. He's also fun in Incusions with the Power Refund. He's also a beast in the new Cavalier EQ as it gives additional prowesses, my 5*R3 was clearing lanes in my exploration. I just R4'ed him today.Oh, and KT1 did a Labyrinth of Legends showcase with him. He butchered LOL.