The Nintendo Switch doesn't come with a lot of onboard storage, which is why you'll want to buy the best SD card for Switch storage expansion. Good job that we're into Amazon Prime Day sales, too, as those microSD cards are even lower-priced right now – whereas the best Nintendo Switch console deals haven't really surfaced.
The original Switch offers 32GB storage; the newer Switch OLED model doubles that to 64GB. Either way, with system information using up some of that space, once you start downloading a game or two you'll quickly use up the space – a download averages around 8GB, but can be over 30GB.
Picking an SD card for Nintendo Switch requires some considerations (I'll detail 'Class speed' at the base of this article). Also don't buy anything that seems too good to be true: there are plenty of reports of fake cards being sold on Amazon from not-reputable brands. So stick with our recommendations below!
Nintendo Switch SD card speeds and classes explained
Note that it's actually the microSD format that Switch accepts, i.e. not full-size SD cards. That includes microSDHC and microSDXC up to 2TB in capacity, as directed by Nintendo on its official site (although no 2TB microSD cards exist at the time of writing, it's a theoretical maximum only).
The Switch supports UHS-I type cards (circa up to 100MB/s speeds), not the newer and faster UHS-II type.
However! There are also U1/V10 and U3/V30 types, the latter proven to be faster for install/loading even on Switch. Look for the 'bucket' U-shape with the number '3' inside it on the card as a visual identifier.
There are also Application Standards, A1 and A2 (expressed literally as so in a visual form), the latter far faster with read/write. While either is compatible with Switch, the console won't take advantage of A2, so don't overpay for this feature.