The two most commonly used buffers for general protein extraction are RIPA and NP-40.

Both RIPA and NP-40 are suitable buffers for whole-cell lysate as well as membrane bound proteins. 

RIPA is the buffer of choice for mitochondrial and nuclear proteins because its harsher properties make it better suited for proteins that are hard to solubilize.  

Tris-HCL is the recommended lysis buffer for solubilizing cytoplasmic proteins. RIPA lysis buffer is fully compatible with a range of individual protease inhibitors and cocktails and may also be used for solubilizing cytoplasmic proteins. However, it is the second choice because it does not contain protease inhibitors.