Perhaps the brightest human in recorded history.
IQ crudely estimated 250-300.

	 :	"... knew at least 200 languages and could learn a whole language in one day."

I'm always cautiously suspicious of claims like this. Who can assess his abilities to converse in 200 languages? How can one learn an entire vocabulary in one day?

While it is plausible that William Sidis was "the brightest human in recorded history", specific claims like this are best taken as indications. There are reports of jesters juggling 17 bottles in the 1600's, but we're pretty sure that's not true. We're also pretty sure that PierreDeFermat did not prove FermatsLastTheorem.