Any standard grease would I think, maybe even a bit of copper slip perhaps?
We use a grease at work that's blue in colour, it a to work with, but it's brilliant at 'holding on'....
It doesn't fling out of fast moving bearings, or ooze out of joints either. I can't think of the proper name of it, but try and hunt that out as I would not recommend 'lithium' based grease....
It's too thin IMO e.g. washes and wears out of where you apply it dead easily, which is why we changed to the blue stuff.
General Purpose Lithium based grease is fine. Only a half teaspoonful will do the job, and wipe off old grease that will hold dirt and grit first, from the coupling as well as the towball.
Copper slip or copper lube is not a general lubricating grease but more a antiseize grease for nuts and bolts or on hot exhaust parts as it tends to dry out and leave a copper deposit behind which prevents seizure. WD 40 make a ptfe grease which provides a good clean type of lub comes in a spray can and resistant to gathering dirt and grit after it deaireates it leaves like a slippy plastic coat behind. a can should last about 2 years I use it on my corner steadies. WD 40 industrial grade PTFE lubricant.