Thanos
Thanos possesses incalculable physical power even in his base form. His strength has proven sufficient to battle Thor, the Hulk, and Captain Marvel simultaneously. His durability allows him to survive direct hits from weapons capable of devastating planets. His energy projection capabilities can vaporise opponents at range.
With the Infinity Gauntlet, Thanos ascends to omnipotence itself, capable of erasing half of all life with a finger snap. Even without such artefacts, his technological resources include world-devastating weaponry, the Black Order's formidable abilities, and the Chitauri and Outrider armies numbering in millions of combat-ready soldiers. Power at this scale renders most opposition meaningless.
The Joker
The Joker's physical capabilities remain entirely human. He possesses no superhuman attributes, relying instead upon conventional weapons, chemical compounds, and theatrical implements of destruction. Batman has physically defeated him in virtually every direct confrontation, requiring moral restraint rather than superior force to avoid lethal conclusion.
His power derives entirely from psychological impact and willingness to employ tactics that conventional opponents will not anticipate or match. Joker Venom, explosive devices, and elaborate death traps represent his arsenal. Against cosmic-level threats, these weapons amount to irritants at best. No amount of cunning compensates for the fundamental mismatch between mortal and titan.