A while back Al broke down the specifications of SD cards and what each class meant and the proper usage for them.  The top class is U3 and it offers transfer speeds high enough to support recording 4K video on your devices and that happens to be the rating on the new 256GB Samsung EVO Plus MicroSD.  Legion Hardware just tested this MicroSD card and it now holds the title of best performing SD card they have tested.  The performance does come at a premium, the MSRP of the card is $250 and even with a 10 year warranty this is still an expensive purchase.  If you need the ability to record 4K video immediately this is the fast solution available but if you are still in the planning stages, remember that there is a new standard, UFS, which is due to hit the market soon and impact pricing of older products.

"Running out of storage on your smartphone, tablet or 4K video camera? Well the good news is SD cards have never been more affordable and crucially offered such huge storage capacities. In fact, Samsung recently announced the availability of a 256GB version of their popular EVO Plus MicroSD card series"

Here are some more Storage reviews from around the web:

Storage