M.2 drives can have different sizes (the smallest is 2242, then 2260, finally the biggest is 2280; your mobo accepts all of these formats. Normally, consumer grade M.2 drives are 2280), and different interfaces with the CPU (either SATA3, which is good, or PCIe which is even faster; your mobo accepts both). [22 is, in mm, the width of the drive, and 42/60/80 is also in mm its length]
PCIe drives are quite faster than the SATA ones, but they are also more expensive. So if you want the best M.2 drive, it's clearly either the Samsung SM951 or the Samsung 950 Pro (they are very much comparable, pick the cheapest one of the two). Here you have a list of recommendations:
http://www.tomshardware.com/reviews/best-ssds,3891.html
Hope it helps!