What type of tactical sling do you use for your AR-15/M-4 and why?

Parachute cord or dummy cord. Never used a sling in SOF.

I like and use my one point sling. It allows me a bit more flexibility and quickness of Point and shoot.

they are a lot of fun to shoot

This one from Viking Tactics. I don't believe it gets better than this.
https://amzn.to/2UF0Kwf

I do not use a sling very often (tactical or shooting). I have tactical sling that came with the Sig Sauer 516 I bought a few years ago. It attaches to the Magpul furniture I have on the rifle. It does the trick but I seldom shoot with it attached.

Here is my take on it. I have single point, 2 point and 3 point slings.

A single point sling is for a person who mostly has the weapon in their hands, but needs to maintain control of the weapon, think a swat operator entering a building who then has to cuff a suspect. Single point slings allow the weapon to smack you in the shins or nuts. But they do allow the most freedom of movement.

A two point sling is for a person who needs to keep their weapon immediately available to them, but also needs their hand free for other tasks, Think a soldier standing a guard post or directing traffic. A two point sling tends to restrict movement a bit more than a single point and depending upon how it is set up may make it more difficult to transition to the off shoulder.

A 3 point sling is like a 2 point sling with needless complexity.

For the use you describe riding a 4 wheeler or tractor I think a 2 point sling would be a better option.

But slings aren't that expensive, I've bought single point bungee slings for as little as $15.

Magpul ms3 is good and gives you 2 or 1 point carry options.
Blue Force Gear Padded Vickers sling is very customizable... and padded.
I also like the Northeast Tactical Bungee 1 point sling.

Single point.

Specter gear makes a really nice single point and 3 point single depending on your preference. The 1 point has a quick detach for those times you want to just unclip.

For pop-up and swing shooting, the single point jobs are usually the ticket. For going out to the tree stand, whatever is comfortable. I have a couple different ones for my AR platforms and use them depending.