Currently the fastest memory you can get would be HBM (1) and GDDR5X. And while everybody thinks a transition to HMB2 is the next and only step for graphics cards you need to realize, GDDR memory isn't dead yet. Samsung is working on GDDR6. It's expected to be released in 2018 and can do 14 to 16 Gbps.

To compare a little, your current GDDR5 memory on a Radeon RX 480 /  GeForce GTX 1070 goes is roughly 7 to 8 Gbps, and GDDR5X on a GTX 1080 can do 10 Gbps right ? Well, that is going to take massive steps forward. GDDR6 when released will start right where GDDR5X will end, at roughly 14 Gbps. The technology is expected to have been advance to 16 Gbps by the year 2020th. This means more bandwidth (double over current bandwidth) and higher clock frequency memory. Volatges wise GDDR6 will be rated at 1.35v, which is exactly the same as GDDR5x.

The new information was posted by computerbase who derived the info and slides below at the Hot Chips 28 event. Samsung is also hard at work on ddr5 and laptop lpddr5 memory, also expecting a release in 2018. 

Samsung to offer GDDR6 at 14 to 16Gbps Bandwidth - Launches 2018