coldmold,
I am with Tim on this, 460 CP, 420 BTB, and 520 BTB would be my choices. The 460 is very accurate from my guide gun. The 420 is accurate from every gun I've tried it in. The 520 definately out punches the rest of them, but shoots about 6" lower at 100 yards from my guns. My loads are "downloads" from the max listed because I figure once you get something that heavy moving along, a 'lil old buffalo isn't going to stop it. My penetration tests support this wild idea.
Have a great hunt and post some pictures please. I think buffalo are great looking animals, and good eats too.
Grizz