(This guide does not include shotgun ammo types)
So, Far Cry 6 ammo types. There is many. Is regular, soft-target, armor piercing, incendiary, blast and poison.
So. This gives obviously the question, what one is best? which one you should use? etc etc.
And this depends of many things. And here you find the answers.

First. You gotta know what all of the ammo types do.
Regular: Regular bullets. They are.... bullets.
Soft-Target: Soft-Target rounds do extreme damage to NON-armored spots of enemies. And very little to armored spots.
Armor Piercing: Armor Piercing rounds do extreme damage to armored spots of enemies. And very little to non-armored spots.
Incendiary: Incendiary is like... shooting fire bullets.They set up enemies on fire, and... well anything you shoot!
Blast: Blast rounds do little "explosion" on impact. Great agaisnt vehicles. Not so great against enemies.
Poison: Poisonous rounds makes enemies poisoned.

Now, if you want the most efficient rounds, you can forget regular, poison and incendiary fully. Regular rounds are useless. Poison and Incendiary are fun to use, but they are not that good. Just use for fun.

And what you do need/want. Is Soft-Target rounds, Armor Piercing rounds and blast rounds. Because with these 3, you can kill/eliminate any enemy/vehicle you will face.
I would recommend having gun for each of these bullets. I personally use Decontaminant (aka M16 A1 but modified/overclocked version) with Soft-Target rounds.
And Eldritch Truth (Aka PPSH-41 but modified/overclocked version) with Armor Piercing bullets.
And then you should have some assault rifle or submachine gun, or even light machine gun if you want. With blast rounds (or just straight away rocket launcher or la sorpresa. Or whatever. To destroy enemy vehicles. Expecially helicopters!!!!!!!

BUT, if you only want or need to use one bullet type. Or you want to know which one is best to use. I will tell it now.
I would recommend Soft-Target rounds. Why? Well. The enemy types of this game, are many. And 90% of them, are non-armored ones. Or lightly armored. There is only ONE enemy who is fully armored. So when thinking this way, it is of course most smart idea, to use bullets, which kills enemies, you will face most of times. Sure, the lightly armored enemies does have little armor. But even then, hitting their face, or body parts that are not armored, kills them with one or few shots. IF you use the Soft-Target rounds.
While if you used Armor Piercing ones, you would only kill all enemies if you hit them on head. And you dont always get to do that. That is why, Soft-Target rounds are the most worth rounds to use.

(To know best rounds agaisnt each enemy and vehicle and etc, check the photo on this guide)