Not all memory cards are made equal. For starters there are different designations to SD cards which denote their sustained speed, along with maximum speed potential. Second, there are two sizes of cards: full-size SD and the smaller-scale microSD.
We've dug into all the background information you could need here: SD cards: Jargon busted, speed ratings explained
So which do you need to buy and for what task? That'll affect which makes the best sense to purchase, so you don't overspend, avoid the fakes, and end up with the correct-sized card from a reputable brand in your digital camera, phone, portable console, or whatever other gadget you have that requires the expansion.
Best SD card / microSD: On the cheap
What's a tenner, eh? You can get SD cards and microSD cards for under that price, without too much compromise. No these don't have the very fastest read/write speeds going, but they're a great quick expansion of data without blowing the bank.
Best SD card: For your camera (UHS-II)
Some modern digital cameras have got on board with the latest in SD card technology bus speed, known as UHS-II. Flip one of these SD cards over and you'll see a rather different pin arrangement from a standard SD card. In the image above, II is on the right.
You'll need to check in the specification that the camera is UHS-II compatible, but if it is then you'll get the fastest possible read/write speeds, enabling you to make the most out of burst shooting without the buffer getting clogged and slowing you down.
SanDisk is a reliable name and makes one of the fastest Class 10 cards going - without you overpaying for a higher-yet designation that you'll only need for Ultra-HD video capture at high bitrates (such as V60 and V90, options below).
Best SD card: For 4K capture (V60)
When it comes to 4K video capture there's a lot of data passing onto the card, thus a V60 (that's 60MB/s) designation is recommended. You could probably get away with a V30 option, depending on capture bitrate, but our solid (and affordable) 128GB option is from Lexar below:
Best SD card: For future-proofing / 8K video (V90)
If you want the fastest of the fast then you'll be wanting a V90, which guarantees 90MB/s sustained write speed. That might not sound as big as many of the other claims you're seeing on the front of such cards, but this is sustained rather than maximum fluctuating potential.
Lexar's Professional range makes a card that ticks all the boxes: UHS-II, Class 10, U3, V90. At the time of writing that's the highest designation possible, making it suitable for 4K video and even 8K. You might want more than one, though, given the 128GB maximum size.
Best microSD card: For your Nintendo Switch
Ok, so this one is a no brainer because, well, it has a Super Mushroom on the card. Made by SanDisk, officially endorsed by Nintendo, and with enough memory (128GB) at a sensible price, this is the ideal expansion card for your Switch to download all those games and goodies (and, er Goombas?).
Best microSD card: For your mobile phone
First thing: make sure your phone has a microSD card slot, as not all do.
Second: don't worry about super-fast card speeds here, because you can keep apps on the phone's internal memory and designate other content, such as downloads, movies, pictures and lesser used apps, onto the mSD card.
HP makes a microSDXC card, at 64GB, that's perfectly fitting for the task. It's also waterproof, should your phone take an unwanted dunk in a puddle.
Best microSD card: For maximum memory (1TB)
The amount you can squeeze onto a microSD card has continued to increase, with 1TB cards now available (and 2TB due in the future). They're not cheap, of course, but if you want maximum memory potential then this is the way to do it.
However, don't buy an unknown brand name as, in our experience, the so-called "1024GB" cards from various online retailers have far lower capacity and are, therefore, a fake/con.
Here is a reliable 1TB microSD option from SanDisk. It'll cost you a fortune, but if biggest is best for you then it's the way to go.