The Beretta M9 and M9A1 is the official sidearm. The Colt M45 CQBP is the sidearm the MEU SOC and MARSOC use.
You’re venturing into hazardous waters anytime you start a gun thread here.
But just to get the ball rolling, I believe the Corps uses the Beretta 92 as their issue sidearm.
I’ve seen USMC embassy guards carrying .357 revolvers.
Glock is also excellent, as is any SIG.
Many love their Smith and Wessons and/or Rugers.
Personally, I go for the 1911 .45.
By the time this thread reaches 200 posts, you’ll see that everyone has a different answer.
Very few of which will specifically address the question as written.
josephm20 had the correct answer immediately after your post so I’m adding some other info for improving your gun lexicon. Glock’s model number system begins at 17 and there never was a 9, except in the sense that numerous Glock models are made in the 9mm caliber. As to the “Glock 18,” it is a select-fire handgun, capable of fully-automatic operation. There are many “war stories” surrounding it but it’s safe to say there’s likely never been one in the hands of more than a very few special ops Marines, if ever.
I got bit by a squirrel once.
It was carrying a glock
“Uniform demerits”? R.O.T. C., you know, really stands for “Remedial Officer Training Course”.