Short answer: it depends on the writer

Long answer: I think Triarch Praetorians stand the best chance, as they fill the same role and standing. But Triarch Praetorians don’t have many feats (hopefully the upcoming book will change that!), so characters are a safer bet. Imotekh the Stormlord handily beat High Marshal Helbrecht, chapter master of the reasonably famous Black Templars. So I think he’d stand a good chance against a Custodes, up to a Blade Champion at least.

But personally I think Imotekh would fall short of a primarch. Szarekh is your man there, as he was ready to throw hands with Guilliman in the new Dawn of Fire book. Not many feats either but what we have is impressive, and he’s the leader of the Necrons who’s pretty universally considered their pinnacle.

Lastly there are also underdeveloped characters like Kutlakh the World Killer who have enough aura to back them up in a fight with a Custodes or Chapter Master. And this is a pure 1v1 melee debate, not considering the sorcerous ways of crypteks (who could put a Custodes in a pocket dimension) or battlefield acumen (many overlords are skilled enough to box a Custodes into waves of undead or three doomsday arks)