The 64gb 633x Micro SD card.
The 633x cards are speed rated at Class 10, and leverage UHS-I technology to deliver a transfer speed up to 633x (95MB/s). This makes it perfect for filming at 2.7 and 4k. Although marked as UHS-I they have the same transfer rate as the new 633x UHS-3 but are way cheaper. The card has been tried in various action POV cameras including my GoPro 3+ Black Edition, Contour Roam and Garmin Virb. In all cases it has been faultless.