Interesting question
 

I agree with Unique for handguns and shotguns and IMR4895 for rifles. All the best...
Gil

Having tried Unique in three different pistol calibers and been very disappointed, I now use Bullseye almost exclusively - the exceptions being Lil Gun for .22 Hornet and .357 Magnum.
RedDot is my Number Two 'go to' powder. My smallest pistol caliber is 7.62 Nagant, the largest is .45ACP. Either powder does them all.

For rifles, I am currently really liking Ramshot's TAC. Loads nearly all my rifle calibers. For the milsurps, I still prefer the IMR4895 though. Some 'odder' milsurp calibers don't have data yet for the TAC powder.

Trail Boss. Yeah my loads would be mild, but I have yet to find anything it couldn't me loaded in and wasn't accurate in.

GoodOlBoy

Gold Bond Medicated

Powder
 

Hhehehehehehe. Good one!! ya coulda said cocaine!!

Varget for rifles.
Unique or Bullseye for handguns

One powder? ok,I'll bite.It's not efficient but will work in all firearms....BLACK!!!They can't stop you from using it if you have access to alder and willow[easy for me],quite a bit of work,and you can make the stuff yourself.Go figgure.