Best mutant to R4?

Chuck_Finley

Looking mainly at dual threat BGs and short form content.

TotemCorruption

Well, along with Sinister, those are the top three Mutant champs out there, especially for dual threat.
Jean is a better defender than Onslaught if your opponent doesn't have reverse controls counter.
But otherwise Onslaught is a better defender by far.

As far as attack, they're similar, can both counter thorns (since Jean is awakened).
But Onslaught needs to land those unstunned heavies constantly, whereas Jean just needs to land one.
You really can't go wrong with either.