Which muscles are most pertinent to fighting in a boxing match?

It’s not one thing it’s a combination of things core (not abs but whole core) legs back forearm ( for stability while punching ) and good grip strenght can help too

 

Its the muscle that Orchestrates the posterior chain everything works of it,

 

I dunno there probably is one but not sure its that important to know or think about, knowing how to box and good cardio important rather than developing a certain muscle

 

In general, you only want enough muscle to protect your body. You don't want to be too big because then you get slow and stiff. (I am aware of certain exceptions to this rule, *cough* mike tyson *cough*, but for the majority it isn't good)
A boxer typically wants to have lean muscle, not bulky muscle that you get from weight lifting. Doing natural bodyweight stuff like pushups helps with this.
Having big muscles doesn't mean you have a big punch, necessarily. Power is something you either have or don't have, like speed. You can work on developing your power and speed, but at the end of the day it's a natural gift.

 

I forgot to mention that you want strong legs. So that is probably the most important muscle, legs. (calfs, thighs)
Ever seen somebody KO'd and struggle to get up because of their shaky legs? Their legs weren't strong enough. Don't let it be you! I would recommend bodyweight squats or using a leg-machine.

 

You want to develop your posterior chain.