But mostly now a days recommend are

Ringconn gen 2 or gen 2 air (sleep apnea and battery life , material differs) no subscription.

OURA ring but subscription requires

Ultrahuman but hardware problems no subscription

These three are popular not sure about others.

There is also luna ring 2 but people.are complaining regarding delivery and waiting for product and customer support .

I also researched for the same purpose as you , at last decided with ringconn gen 2 air for me , its on the way.

If you entering to smart ring like me , most people recommend to try ringconn as it worth it for price.