I mostly use the one called “Clinometer “ on my iPhone. It uses the camera in combo with the sensor. Nice thing is, it goes straight to Clinometer, so no need to waste time clicking through other options.
It speaks the result (nice if you have bright sun on your screen.)
I also use the pole sticker, I get pretty good results, and quickest to deploy, but I have to practice more with it every season. I tend not to trust it as much, but that is mostly operator error. I tend to use it a lot at a quick glance, and if anything is critical, I verify with another Clinometer.
Weighs nothing, costs almost nothing, can’t break, can’t be forgotten. I think everyone should have one.
the ski pole inclinometer sticker
poleclinometer.com
I keep my mirror compass in a zip pocket on my waistbelt. It is much harder to read than the phone, but I can use it with any gloves, and it doesn’t matter if it gets wet or cold, no batteries to run out of.