What prefix would be best for adding to mortal make it the opposite?

Here are some prefixes you could use to make "mortal" the opposite:

* Im-: This prefix implies a lack of something, making "immortal" mean "not subject to death." This is the most common and direct opposite.

* A-: This prefix also indicates negation, so "amortal" would also mean "not mortal." It's less common than "immortal" but still grammatically correct.

* Un-: While less common, "unmortal" could be used to convey the opposite of "mortal," implying a state of being not subject to death.

Ultimately, the best prefix depends on the context and the desired tone. "Immortal" is the most widely recognized and understood term for the opposite of "mortal."