Q:If my opponent casts Black Sun Zenith, and in response i cast momentous fall, would i be able to keep my creature, or would the black sun zenith resolve after the persist does.
I’m assuming the creature you’re sacrificing to Momentous Fall has persist. If so, persist will trigger and go on the top of stack.
Objects on the stack always resolve FILO (first in, last out), so persist will resolve and return the creature to the battlefield before Black Sun’s Zenith.
Finally, Black Sun’s Zenith will resolve (and presumably the creature will die again).