I mostly load pistol cartridges, but I'm gonna say IMR 4895 because when I do load rifle, it takes so much powder. An unlimited supply of free rifle powder would really get the price down. (and I probably already have a lifetime stash of pistol powder)
If someone was going to give me a bunch of powder and said it had to be pistol powder because that's what I use, I think I'd choose Vhitavouri N330.