Standard buffer or Heavy 2 stage buffer.
You can get other ones like the MSG90, PCS Heavy Buffer, or the Bill Springfield buffer.
HK 91, G3 - Stocks, End Caps, Braces - HKPARTS.NET
I have the HK 2 stage Heavy Buffer in my SG1 stock on my HK91.
I use the PCS buffer in my SBR V51.
They both are very effective at taming the recoil.