If in doubt, simply use PBS. That should keep all proteins happy. If background is high, add 0.1% triton or tween. If you need a preservative, 0.1% sodium azide should do (mersalyl if you detect by peroxydase).

Valeriy,

why not examining the dependence of the primary antibody (or its conjugate you use) activity on pH? Then make a choice for a buffer.

Good luck!

P.S. Let say, pH 5 (acetate buffer), pH 7 (phosphate), and pH 9 (borate).