The Collective? The Void?
Who the most powerful foe he has defeated?
I need actual fights in comics not hear say on who he supposedly best in his past life.
Molecule Man, Absorbing Man, Adaptoid, Terrax, WWH, Ultron, Blue Marvel, the Inhumans, himself, Attuma and his army, Carnage, and many others that I can't recall at the moment.
And of course the Void has pretty much faced everyone at the same time and looked good doing so.
An important thing to rememeber is that his power is limited by his emotions so you could have him fighting the Thing one minute and a team of heralds the next. I think that since his return we are only now seeing him at his firt mini levels because he believes himself to be taking the serum again.