I have been collecting Lexar SDXC 2000x UHS-II/U3 SD cards, which claim to have read speeds "up to 300MB/s." These cards use an extra set of pins on the back to achieve such fast transfer speeds, but there's a catch: your cameras and card readers must be compatible with UHS-II in order to take advantage of the increased speeds. Without explicit compatibility, these cards perform no better than inexpensive SD cards.
Some high-end cameras, like the Sony A7R II, can't take advantage of fast SD cards. In this SD card speed test, the Sony never exceeds 35.27 MB/s, which means that cards like the Lexar 633x UHS-I/U3 SD card, which are less than half the cost, work just as well (and also support 4K video in Sony cameras like the A7R II and RX100 IV/V). This Lexar UHS-I/U3 card is starting to be hard to get, but Sony's equivalent card is a few dollars more and also works well.
I am currently on assignment in London capturing about 200GB a day in still images. Copying and backing up such large amounts of data is a huge pain, and the built-in SD card slot in my MacBook Pro was getting an average of around 45 MB/s when reading from the 2000x. Switching to the $8.95 Kingston Digital MobileLite G4 USB 3.0 card reader, which supports UHS-II cards, tripled my read speeds to 140 MB/s (2000x SD card) and 100 MB/s (1000x SD card). This turns 75 minutes of copying into 24 minutes—an amazing time savings for a $9 dongle!
The Lexar SD card reader that comes bundled with 2000x-speed cards is also supposed to be even faster, and the standalone Lexar SR2 card reader is supposed to be the fastest, but it's relatively bulky when used outside of the Lexar Workflow Hub, in which it was designed to dock.
If you're using fast media cards, make sure you have the right accessories to take advantage of them!