Just the system embedded LOS alarm clock. I don't trust any third party alarm clock apps to work when I need them to.

I use Sleep as Android for:

1. Great softer sounds that can be adjusted to gently increase alarm audio. I use bird sounds.
2. It can control lighting too.
3. It's smartwake looks at my sleep stage to choose the best time to wake on the window of time I allow. It uses my watch to measure sleep stages, but works with numerous other watches, chest straps, etc. or also has its own Sonar function using ultrasonic to detect stages from movement.

But chiefly it also has other stuff I use apart from the waking alarms such as sleep stats, warning for bedtime, lullabies when going to sleep, anti-snoring measures, noise recordings, 30 min power nap management, etc.

None. Just the one built into Samsung.

Simple Alarm Clock.
Extremely reliable during the ~10 months I've used it so far and it is indeed simple enough.

Clock You. The You suite has some great apps, I use almost all of them. Not only are they libre but they have attractive and very useful GUIs.

On which platform?

AMDroid. It's the only app I can find that allows me to create alarms from calendars.

Chrono or fossify clock

New Clock

Downloaded with Obtainium.

It is new, elegant and supports direct boot, meaning it will wake me even if the device auto updated from an update over night.

Clock You or Fossify Clock