1. I recommend SanDisk. Samsung is probably just as good, but SanDisk invented the technology and are always on top of it. (When Samsung released an update that didn't handle the SD card properly, SanDisk came out with a modified card that would still work - before Samsung had even acknowledged that there was a problem.) They're also a very easy company to work with, have an on-line chat manned by knowledgeable people, answer emails promptly (like an email I sent on Sunday night that I received an answer to by 10 AM Monday morning - and that was 7 AM their time).
2. Samsung and SanDisk make the chips that are inside the card. Everyone else buys from them. Are they always getting the best chips? Who knows? And if there's a problem who do they blame, themselves or the chip manufacturer? Samsung and SanDisk are "single-source" - if it doesn't work, it's their fault.
3. Up to HD video, a Class 10 (or U1 - same thing, different nomenclature) is fast enough. For 4k you need a U3 (there's no "Class 30").
I've been buying SanDisk since cards were measured in MB (I still use a 256MB card all the time for small files, I used to have a 32MB card, but darned if I know where it is, but it worked the last time I used it) and the only one I ever had a problem with was one I cracked. (The 256MB card is WAY out of warranty, but it still works.)