As many have said, no trainer should need equipment; but I’m pretty sure you already know that, given your question.
If I personally had to choose one piece of equipment I would probably choose the ViPr. It has so many varied uses as a means to add load, momentum, etc. to challenge (and in turn enhance) movements (i.e. function), which at the end of the day is the only reason we use equipment. Plus, due to it’s shape and varying hand positions, the tool itself has built in progression (e.g. ability to change the lever arm length) as opposed to say a dumbbell or resistance bands. I also believe it to be more authentic in regards to the drivers of movement than the TRX, though of course the TRX is an absolutely fabulous tool that I use often.
Anyway, that’s my two cents. Good luck!