Amazon is selling the Samsung 64GB microSD card for $22.99. This is a good deal considering the blazing fast 100 MB/s read speeds this card is capable of. I don’t know if the improved speeds that this UHS 3 rated card is capable of would make a difference on the Fire TV 2, compared to an average microSD card, but if you’re storing apps on external storage, it’s always best to use as fast of a card as you can to reduce load times and improve app responsiveness. That’s why Amazon recommends USB 3.0 drives for the Fire TV, even though it only has a USB 2.0 port. This microSD card also comes in a 128GB version for $44.99, but those are in very high demand so are backordered for 1 to 2 months. If you’re interested in the 64GB version, get your order in quick before they too get backordered.
The traditional speed ratings are only relevant to video recording. They don’t tell you anything about the random access performance. Look for cards that are rated A1 (or A2).