Binary Eye
It's a QR/barcode/etc. reader. Very simple, and after trying like a dozen different QR readers I've found it to be the most reliable.
I know that a lot of brands' built-in camera apps can read QR codes nowadays, but I find a dedicated app faster than the all-purpose camera apps I've used, and if you use open-source camera apps like AOSP's or OpenCamera, then you won't have that feature built in anyway.