Alright two senarios first Champion has the power gem second Champion with out the power gem. Thanos in both fights are in the mindset that he was in when he fought Odin although this is a fist fight.
champ without the gem was taken down by She-Hulk. No way he gets anywhere near defeating Thanos.
With the gem he still loses- Thanos' has defeated him before with the gem without landing a shot. (by letting his shields absorb the damage until champ destroyed the planet, essentially BFRing himself.) He's clearly outclassed here.
Thanos has also manhandled THOR with the gem before, and Thor's powerset and fighting skill are so far past champ it's hilarious.
Essentially it boils down to the fact that Thanos' durability is incredibly high, enough to hang even with gem holders without difficulty (he's done it to a gem equipped drax, too) and Champ is an idiot that doesn't really know how to use the gem to its full potential. Neither version of champ gets anywhere close to defeating thanos, even h2h.
Champion also defeated Colossus Sasquatch and damn near killed Thing and he was just boxing with them. He had Colossus busted up on the inside and I don't believe Champion had the power gem. He also beat Mantis in combat as well.
champion's first appearance was the pinnacle of of his on panel career, he was weakened by the writers in every subsequent appearance after that culminating in the she-hulk bullshit.
nobody is a better fighter than the guy who has devoted at least billions of years to perfecting his body and fighting abilities by willingly traveling to planet to planet, defeating their champions and appropriating their fighting arts.
he totally channels the power primordial into his physique rendering him totally immortal and indefatigable.