I like the pistol grip Lincoln gun. Lucas Red n' Tacky grease has served me well. But I think you will find opinions on grease are about as volatile as opinions on motor oil!
Are you off roading a lot in damp condiitions?
We get into a lot of swampy shit here. Goo/water and more goo and more water...
Marine grease works great.
Ultimately this isn't something that needs a cigarette and coffee to chew on...
Mobil 1 we have cases of...
But other stuff works fine too.
The key is not neglecting maintenance!!!
Bacon lard is better than no grease and runnimg dry.
I use redntacky - but can’t say it’s the best with any confidence, the only advice I have on a grease gun is to get one with the quick release lever. I have to unscrew mine and with grease all over that becomes futile quickly.