So I commute year round in Calgary and that means cold and dark. I can't seem to find a tail light that isn't disposable garbage and will actually hold up to a few years of use.
Yes you can basically buy tail lights in bulk off Amazon - but I hate tossing dead junk in the trash. I would rather pay more for a light that lasts.
My perfect light would use replaceable 18650 (or whatever size) lithium ion cells, or at least have USB-C with pass-through charging (so that I can run it off an external battery in a pinch).
For front lights I use Outbound Lighting and they are perfect - long battery life, great performance, pass-through charging, works in the cold. Unfortunately Outbound doesn't make tail lights. I've begged.
I have a few Knog lights made of silicone that don/doff easily from the bike. Very convenient, bright, and water resistant. Unfortunately, I don't see the tail light model I have (Knog Skink) on their website, but that's probably because it's so old (almost 15 years old now, still working great). Their new tail lights look pretty nice and seem to be affordable.