I am a stickler for genuine canon flash, I have third party other things, but for speedlite I think it needs to be canon, so many extra functions and features that integrate seamlessly if you keep it all canon, for me flash is so full of pitfalls and brickbats that the simpler the better.
I do have an off brand ringflash, but it's a purely manual affair and was very cheap.
I have third party lenses, sometimes it makes sense to stick to canon, other times the third party manufacturers plug a hole in canons range, such as the Tokina fast UWA 11-16, the bright standard 30mm f1.4, I've also owned the Bigma in the past (Sigma 50-500)
I find with sigma there is more sample variation, I got a 17-35 f2.8-4 which was horrible, swapped for 17-40 (eventually swapped for kit 18-55!! now using Sigma 18-50 f2.
I bought a lensbaby, thought I would use it lots, haven't really.
I would say for the more esoteric, get hands on in a shop or ideally hire before committing.
I bought a 24mm TS-E (mk1 as was available at the time) wanting to do lots of fake minature, but never really did, and so sold it. At a modest loss, to be fair.
I use calumet and energiser batteries, no issue. I use Hoya Green label filters, no issue. I use chinese intervalometers, no issue.