I personally wouldn't go less than .308 but a mate of mine head shoots "quiet" Buffalo that we are culling.

He is a damn good shot, knows where to shoot them and is quick - as in shot the mother and two calves in a few seconds although once you shoot mum, the calves tend to hang around.

So I would say 270 / 6.5 head shot with a good bullet
- behind the ear is the best as a high front on shot will go into an air cavity / hollow.

Maybe a slightly bigger cal for the bigger animals if there are any.

Re the "behind the ear" head / brain shot, make sure if you are close to allow for the scope above the bore otherwise the bullet will hit the base of
the horns and not go in behind the ear !!! I know, I did it with a 9.3 and the horn stopped the bullet dead in it's tracks, making for one shaking head, very pissed off bull that luckily couldn't run / charge as it already had two broken shoulders / legs !!!

Finally, stay away from the jaw - the jaw bone on a Water Buffalo can / will stop any bullet dead in it's tracks and the teeth mess bullets up as well, both not factors you want when trying to get to the brain if you screw up a front on shot.

Hence the behind the ear shot.

Anyway, have fun and good luck.