generator rex wins
I go with the guy who survived a nuke.
Mercer would win. His regeneration has allowed him to survive a nuclear explosion at a pretty close distance. Rex doesn't even have half the destructive power of a nuke so Alex will be regenerating from everything launched at him. And due to the ridiculous versatility and deadliness of Alex's abilities, he should win. Not much will stop him from outright absorbing Rex.
The apes are kinda angry and are thinking they want a planet all to themselves.