You're getting some great advice. I've been on 6 safaris, Botswana (2), Zambia, Mozambique, Zim and RSA. I consider that pretty limited experience as there are so many areas and PH's in these countries that I have not seen and met. I will say that the best buffalo genetics I have seen were in the Upper Luangwa Valley (Zambia) and the Greater Kruger Reserves (RSA).
Tanzania is on my bucket list and Nathan at bullet is at the top of that list.
Most importantly, ask enough questions to make sure that you will be doing a proper tracking hunt for buffalo in a remote bush environment with a true safari camp. You don't want to show up and find you are staying in a public guest house (yes this happens). Also, make sure you understand whether you will be sharing the camp with other hunters and be sure you are comfortable with the answer.
Regarding location, the two most remote areas I have hunted in Africa were the Upper Luangwa Valley (Zambia) and the Northern Kalahari concessions in Botswana. Bots is off the table until they get some sense. Zambia still seems to be sorting itself out after the closure but I do know the fellow who runs the area I hunted, a true remote wilderness area if you are interested. Bullet in Tazania sounds like a proper bush hunt done right and their pricing seems fair.
Zim likely has the most well developed safari industry of the countries I have listed. Zim is truly impressive in its support infrastructure for the safari industry. I am not including the RSA ranch hunts in this statement as I don't think that is what you are looking for. Obviously they are extremely well developed.
Be sure to understand transit costs to and from camp on each option. These can greatly swing total costs from one option to another. This is typically an issue in Tanzania and certainly is in the remote areas of Zambia. Many operators offer a drive in option if you are ok with a day on the road. I personally prefer this as I like to see the country.