What would be the better powder for .40-65, .45-70,& .50-70? I've tried 1Fg in the  .50-70 and 2Fg in the .40-65 but I'm wondering if one is better than the other for the 3 calibers that I'm thinking about going to the dark side to load. I've got great smokeless loads for the  .40-65 and  .45-70 but I'm struggling to find one for the .50-70.

Any of the black powder granulations will work fine, especially in the 50-70.   A lot depends on the rifle and bullet you want to use.   then factor in what your want to use the rifle for and your tolerance for recoil.  I've been using 60 grains of Fg in an original Sharps 50-70 carbine and can usually see 1.5 inch groups at 75 yards from the sitting position with the original sights.  In my 1870 Navy rolling block 50-70 I've found 67 grains of FFg and the 450 grain government bullet to work very well.     I use that same rolling block load in my Farmingdale 50-70 infantry rifle. 
 
In the 45-70 I use mostly FFFg or FFg depending on the rifle and bullet.

You won't find a magic combination without doing a lot of experimenting yourself.

George

I've tried 3Fg and none of them did well with it especially my shoulder and I'm not recoil sensitive.