How can I recruit the most skilled soldiers?

The answers here seem not very good. Having played the game a lot, I've found the following:

Recruit everybody. At the start of the game, you will only be able to recruit E-C level soldiers (C's being rare). Better soldiers get posted based on your own rank.

Fire the bad ones. As you progress through the game, you will begin reaching your population limit. While expanding the base is the right thing to do, as you go, you will need to start setting a limits for minimum skill. Don't recruit anyone at that point below a certain level (unless you have to or they have very special development skills needed to progress).

Do not spend MB coins on people. Do not spend MB coins on completing dispatch missions early. You will need these MB coins to expand your FOB's. Per value, that's the best usage because you can drastically increase your population.

As for acquiring the best people, I've found that the best skilled people are spawned in missions. Bases (OKB Zero, Power Plant, Airport) have the next best. Outposts typically don't have very good soldiers. Sending your team to expand via dispatch missions is also good. And complete all missions with an S rank to get a good supply of S soldiers.

Also, I've only got a basic understanding of morale. The easiest way to increase morale is to keep a high amount of money. Do not spend down to 0 GMB. Only buy the weapons you need. You can purchase most low level weapons to find your favorites, but then expand in a controlled capacity (max out 1-3 weapons). Don't go into the field armed to the teeth. Control your support costs by customizing a low level weapon with required upgrades. You spend a ridiculous amount of money taking out high level weapons, and usually a well-modded low level weapon can do everything a high level weapon can.

Specialists, at least with a moderately high level, do not have guaranteed higher stats. Hats indicate classification of soldier, most primarily what weapon they will use, but not necessarily their skill. Mark all enemies and find out their skill. D-Dog is helpful when kidnapping an entire base.

Recruitment is a slow process, and the game seems to limit how fast you can raise at the start.

There is a way you can encourage more strong units to appear, when you perform missions, replay a mission you have alread completed, and go in Loud, get seen, shoot loads, and use grenades, to get the security ratings up.

Next time you start a mission on the map those 5 icons should be white or red, means your more likely to come across a strong unit.

Another two different ways:

1) You can replay Mission 29 (Metallic Archaea) and fulton the soldiers of the airport. Kill all the Skulls and when the mist desappears the will fall unconscious. This soldiers receive a medal for being close to death in battlefield, raising their levels even more EDIT: you can replay the Code Talker rescue too.

2) Buy them on the online rewards with MB coins. There you can trade your MB coins with S or more skilled soldiers

One method touched upon by two bugs in the comments is that you can interrogate soldiers so they mark on your map where the "Specialist" soldiers are. If you're low on fultons, in a hurry, or being very particular, I highly recommend this strategy. Specialists typically have higher stats than their comrades, and there is usually at least one per camp.

You can interrogate soldiers through two methods:

1. CQC choke hold an enemy by pressing and holding the CQC button (default R2 on PS4 controller). Continue to hold this button then open the communication menu (L1, likewise).
2. "Hold up" an enemy, either by approaching stealthily and drawing your weapon at close range, or by drawing your weapon during a multi-hit CQC combo. Again, open the communication menu while maintaining your weapon draw to reveal the interrogation options.

There are two options, one will get them to mark on your map some nearby soldiers you have failed to already mark. The other will get them to mark a point of interest, be it some processed resources, rough diamonds, or their nearby specialist.

It makes sense to just recruit every soldier you find. In the end the difference between an E level soldier, and no soldier is greater than difference between an E level soldier and an S level soldier. There are lots of ways to raise the cap on staff at mother base. I've been recruiting everyone and I still haven't hit the staff cap.