depends a lot on what you already have
This is 9 men Infantry squad. I already choose one engineer.
I have all the infantry weapons in BR3.
AT person
I always set my Rifle Squads to have 2 Engineers, and an MG. Just my personal preference. (I just pretend the 2 Engineers + MG are the Machine Gun section of the Rifle Squad, the Engineers being the “Ammo Bearer” + “Assistant Gunner”)
An AT Trooper would be best though, so you can handle enemy armor when needed.
I always have a rule for myself: each squad must have an engineer and an AT, if it was not possible to have both at the same time, put the AT.
I just don’t follow this rule for fun sometimes, like yesterday I removed the AT from mg39 squad and gave it a second engineer, just for fan!
The assaulter vs MG depends on what level I am playing, and what faction. If Allies level three, I choose MG because I can never have enough BAR’s, while most of the other factions I would probably choose an assaulter. The engineer, AT and Radio squad I usually base off what I need vs my other squads. I rarely choose the AT squad since I destroy far more tanks with my dive bombers, so it comes down to the engineer vs the radioman, and I usually split those amongst all my squads.
I usually pick the options that give me the most versatility per squad (yk, only 4 slots), with preference for support classes (they can do something else than shoot a gun). Usual order is: engineer>AT gunner>radioman>medic>MG gunner>sniper>assaulter.
In this situation, I would pick the AT gunner first and then the MG gunner.