I mean M.2 SSD as main boot drive.
If you have an M.2 socket on your MB, I recommend the SAMSUNG 950 PRO M.2 .
If you don't have an M.2 socket, I recommend the OWC Mercury Accelsior E2 PCIe SSD on a PCIe 2.0 x4 slot or better.
Then get yourself a Sandisk Extreme Pro SSD installed on a SATA III (revision 3.x) (used to be known as SATA 6Gb) and you'll be set for speed