The strongest member of the Phantom Troupe is none other than Chrollo, in my opinion. Though this has not been directly confirmed, I think it is hard to argue other than him. Of course, going into this reasoning is a bit spoilery. Chrollo is the leader of the group but this is not just for show. His Nen power is insanely amazing, allowing him to copy other Nen abilities and use them. This, coupled with his base power makes him the strongest in the group. The other person that you could make an argument for is Hisoka. Or, well, he could have been the strongest member when he was part of the Troupe. However, there is evidence to suggest that Hisoka is second to Chrollo. His loss most recently in the manga in his long-awaited fight against Chrollo makes it clear who the stronger member is right now.