What’s The Most Useful Smart Ring Feature? Vote Now!

Whether you are using a smart ring, or are planning to buy one, what feature you consider the most useful?
Sleep Tracking
Fitness & Activity Tracking
Blood Oxygen Level Monitoring
Heart Rate/Pulse Monitoring
Stress Level Monitoring
Contactless Payments (Credit Card in a Ring)
Contactless Access Badge (Smart Key)
Contactless Transit Pass/Travel Ticket
Two-factor Authentication for Gadgets & Online Services
Motion Controlling (Pointer, Mouse)