The one I purchased is the Wellue/Viatom O2Ring, which for my purposes I am happy with.
The following is not intended as any kind of recommendation, just a record of my thought process prior to purchasing. Your priorities might differ.
I discovered it while websearching for an overnight-recording oximeter with Bluetooth connectivity (O2Ring offers that only for macOS, which is what I use, a special USB cable adaptor instead used for Windows).
Websearching also revealed that finger-worn ring oximeters were generally more accurate/reliable than wrist-only ones. So not an excuse to buy a smartwatch or fitbit etc., which (AFAICT) are more suitable for fitness type use (the O2Ring would wobble about too much).
Importantly to me, it has a vibration-alarm, whose intensity can be configured. I specifically wanted something to wake one up if O2 fell too low.
I liked that it had pretty good reviews, such as
this fairly detailed one
Also it was apparently (to some extent) medically suitable (e.g. "FDA Registered"). Which I guess means it's "not too bad".
Lastly, if I'm honest, it looked sleek, cool and comfortable: a nice change from the "clothes peg" type I had used a few times before. I'm sure there are other makes/models with broadly similar features, this is just the one I went for.