If you could have just one powder......

[Jack]

I'd vote for Unique as the most universal pistol powder. For rifles, the aforementioned 4064, 4895, and Varget would be good choices. So would AA 2520.
I once tried to winnow down the number of powders I used. That project got abandoned rather quickly. I think you'll find that one or more of your firearms just doesn't like whatever powder you've chosen as your universal powder, and that performance is compromised in some others.