For instance, Riemann mapping theorem is more than just incredible: merely using the word 'incredible' to describe it is not enough. So, what is more incredible than incredible?
Paradoxical (seemingly impossible)?
Counter-intuitive - not as string but certainly apt for your example.
And unbelievable - is means exactly the same as incredible, but it may have some stronger connotations because the link to stronger forms of beliefs is more obvious.
You could use an actual hyperbole: impossible!
Inconceivable unbelievable; incredible.
I suppose uncountenanceable means 'not even admitting of consideration that [it] might be possible'. It's rare enough to be self-emphasising. But it doesn't fit as a descriptor for a theory, of course.
Riemann mapping theorem is extraordinary.
Alternately, consider such colloquialisms as mind-boggling and unreal.
mind-boggling: having a powerful or overwhelming effect on the mind
unreal: so remarkable as to elicit disbelief; fantastic; unbelievable
You can use the expression "beyond human knowledge" to give the idea of something really difficult to conceive.
For these cases I normally use "mind-blowing" E.g. "The multiverse theory is just mind-blowing"
I think phenomenal is more than incredible. You might be an incredible basketball player but only a few are phenoms.
adjective: exceedingly or unbelievably great
If you just can't believe something because it is so incredible then it is incredulous.
adjective: not disposed or willing to believe; unbelieving
And in the very rare cases, the word we might reserve for the other-worldly, is preposterous.
extremely unreasonable or silly
Try superlatively incredible.