I know Thanos is a telepathic BAMF, but I severely doubt he's on par with Onslaught.
Also didn't pretty much all the xmen and avengers have to throw themselves into onslaught since he was pure energy?
He was pure energy contained inside a strong set of armor... which needed savage Hulk to be broken.
I remember Onslaught being stated as having reality warping and whatever, but I don't ever remember him using it for much. In the end, the team that defeated him really didn't have anything Thanos himself couldn't put out. Hulk's Strength, Jean's Mental attacks, Cyclops's beams, etc. etc. Plus Thanos is resistant to matter manipulation and has some of his own.
In the end, Thanos is smarter, has better standard equipment, and can match if not surpass most of Onslaught's abilities.
Though I'd ask @killemall how accurate my assessment is, he's the Thanos expert :)
Thanks for reading,
Onslaught may be tough but Thanos is a darn good all rounder with telepathy, matter and energy manipulation, strength, durability as well as standard gear to make Onslaught's well... onslaught pretty much useless. He's ridiculously resistant to having his body manipulated. Onslaught has no way to put him down while with time Thanos will perceive a weakness and exploit it.
Onslaught had Magneto's powers, Pro X powers, Nate Grey (X-Man) powers, and Franklin Richard's powers!! He was able to rag doll Juggernaunt and made him afraid to reveal who Onslaught was!! He was able to use Franklin's powers to warp reality and basically made anything he wanted like a second sun! I do not see how Thanos could even out smart him because if he had all their powers then I am sure Onslaught had their smarts as well...
Onslaught with the power of X-man and Franklin, and going purely by his theoritical potential would stomp. On panel feat, Thanos has much better.
As per him being a being of pure energy, so was Rot.