McCoy
was written to be a little overboard at times, but the net camaraderie between him and Spock does prove they're great friends that tolerate each's quirks and in some episodes we do see how Spock can get fatigued of it all as well ("I, Mudd"'s opening).
I wish we had more of Dr M'Benga on screen, but based on this:
He'd still get my vote. Definitely trained and takes action forthwith, what's not to like? Wish we had more of him aboard. Plus, the use of air filter as a futuristic gauze pad is pretty nifty, not likely readily spotted at the time - just like the liquid spritzer, which was also a new contraption at the time...
Oh, McCoy could have said that if M'Benga needed a backup in a way that Nurse Chapel could not provide, then he wouldn't be able to leave Spock, but Kirk's reply is terrific enough on its own.
Chapel could be written hit or miss, but the acting was never less than excellent considering the material. I'm so used to Majel as Number One.
My apologies for the circus clown music poorly mixed into the scenes but ignore that rubbish and Chapel still has some excellent moments.