I've used 4350, 760, H414, Varget, RL15, and 47 SL in the .375. 77 grains of 760 or H414 with any bullet between 270 and 300 grains works great, 47SL needs 78 to get to the same spot. RL15 works with 270 grains better than 300 and usually gets the highest speeds.
I loaded 300 TSXs and Banded solids in the .375 as a back up buffalo rifle, but shot my buffalo with the .416. Judging by the way the solids whistle through a giraffe that is the size of 2 buffalos I wouldn't be using them for the first shot on buffalo. The chances of hitting 2 are too great. The 300 TSX exited a eland for me, you'd best be looking behind your buffalo with those as well. PHs are split between softs and solids for the follow-up, and all premium softs. Everyone I talked to wants a soft for the first shot.
Randy Brooks of Barnes Bullets swears by 270 grain TSXs for buffalo, and has killed more than most. The chances of the 270 solid shooting to the same POI is good. Swift A-frames have a flawless reputation for the big stuff too, and are a little easier to keep inside. This is one of the few times you don't want an exit hole.
I'm going back with 270 grain TSXs and Banded Solids in the .375, and 400 grain A-frames and Banded Solids in the .416.