i think any feats onslaught did are questionable at best. the whole series was written so badly and disreguareded alot of set rules.
everyone is mentioning what he did to the juggernaut. think about this. juggernaut took thors godforce and not only survive, but survived and was undamaged enough for him to decide to try and actaully walk against thors godfoce. not just take it and live, take it, stay standing and try and walk straight into it. the ground broke through because there was too much power for either force to overcome the other. so thats how durable the juggernaut is.
onslaught however is supposedly strong enough to put two fingers through juggernauts chest. so that means onslaughts two fingers alone are capable of far far beyond class 100 strength. doesnt that strike anyone else as stupid?
even if he used his forearm to do it, that would make his fingers more durable than the juggernaut to not have broken. has hulk ever actually injured the juggernaut hand to hand? hes sent him flying a few times but without upgrades i havent seen him actually damage the juggernaut. but he could damage onslaughts armor, so that would mean the juggernaut is more durable than onslaughts armor, but onslaught still used two fingers to go through juggernauts forcefield and into his equally durable chest.
then the whole psylock reading juggernauts mind with the helmet thing. and the fact that the gem has been very clearly established to reside outside the juggernauts body.
i think thanos could win this. disreguarding his intellect, he has above herald power, and heralds can easily shatter planet, he can take freakish amounts of damage before he even shows signs of weakening, he can teleport, and he is a very powerful magic user.
i read the whole onslaught series and i cant in good concience vote for someone whos feats are so riddled with pis above someone like thanos, whos proved time and time again that he is worthy of all the hype that surrounds him. thanos has actually destroyed the universe. who else in marvel can say that. not some alternate dimension, or someone who can "potentially" destroy it. who has gotten it done as many times as thanos?