If you don't mind charging devices, Oura ring is my top choice as it's the most accurate wearable. Fitbit does a decent job if you're on a budget, but the data is less accurate.
My personal favorite is set and forget, under the mattress - Withings Sleep.
The hard part is making sense of the data and determining what to do to try and improve your sleep - getting a tracker won't simply improve your sleep.
To fix this issue I build SleepWell.ai, that takes sleep tracker data and makes custom recommendations based on what sleep science has shown will improve sleep.