(1) Brand? I use only Sandisk and Lexar cards - I decided to use only cards from top brands after three cards randomly and suddenly died. One expired physically - the plastic became brittle and it literally fell apart! I lost all my images from the ones that simply stopped working.
Warning: Lexar sold their flash card business to a Chinese company last year. So, no more Lexar cards... I will be buying only Sandisk.
(2) Size? In case of loss or failure, I don’t use large cards. I use 16 and 32 GB. One or two cards cover a day’s shooting even in my preferred Raw + JPG mode while using a 36 MP camera (recently replaced by a 42 MP one).
(3) Speed? I use Sandisk Extreme Pro, which advertises 95 MB/s read/write. That’s more than enough for me, for now: my current camera, a Sony A7R II (released in 2015), has a real-life write speed of about 35 MB/s. The new Sony A7R III (released last summer) has write speed of about 150 MB/s. However, I’ll only see a difference between the two cameras - in spite of their massive files - if I shoot in burst mode. But I always take singke images, and never machine gun.
Incidentally, the Canon 5D Mk III and IV have similar write speeds to the Sony A7R II and III.
In short, buying the fastest card will be a waste of money for any camera, and even a fast card is is too fast except for very recent cameras. But see the next point, where a fast (not the fastest!) card is convenient.
(4) Card reader? The bottleneck for me is not the camera but the computer. Occasionally I use someone else’s computer, and their card reader either refuses to see my card or is glacially slow. Most card readers are rubbish! First, make sure that it’s compatible with modern cards. Secondly, that it uses USB 3.0 not 2.0 (the latter tops out at 35 MB/s). Thirdly, many USB 3.0 readers are inexplicably slow (my suspicion is that they’re USB 2.0, mislabelled as a fraud by iffy companies). Google for a brand-name reader that has been tested and shown to be fast and reliable.
NB: you don’t need to spend a lot - my current reader is the tiny Transcend RDF5 (about £6 or $10).