I use a combination.
Mostly *A and credit to UA. That lets me use the miles on TG, NH, UA and LH.
I also have a pretty big balance on Amex, but there are limits on how much you can use on NH and JL. Very good for SQ and a bunch of other airlines.
I've also bought a bunch of AS miles which are pretty handy on EK, CX, JAL, QF, etc.
Depends a lot on where you are going, but between Japan and NA I do about 80% of my flying on NH.