which finger is best for pulse oximeter reading?

Which finger to use in a pulse oximeter? 
As per the studies, your right hand's middle finger shows the best results. 

Fingertip pulse oximeter sensors are one of the most common variations of at-home pulse oximetry devices used across the country. 
Make sure to take off any nail polish and avoid using on cold fingers as the readings may not show correctly.

Designed to make taking your oxygen saturation readings as quick and painless as possible, this method is the best at accommodating a fast-paced lifestyle. However, many who use these devices often find themselves wondering whether they’re placing their pulse oximeter correctly—or even whether they’re getting accurate enough readings.

You need to maintain normal breathing and sit still before performing the test.

When a large, easy to read display is one of your main criteria, the ESPOO fingertip pulse oximeter has to have one of the biggest, and this device gives you nearly instantaneous readings in just five seconds. This pulse oximeter can accurately determine your SpO2 (blood oxygen saturation levels), pulse rate and pulse strength.

It allows you to set the upper and lower limits for readouts on pulse rate and oxygen saturation and features an alarm when your readings fall out of your set range. 

One button control makes operation simple, and it has an auto-shutoff feature after eight seconds. It’s made of durable ABS and the fingertip grip is lined with soft silicone for comfort. We also think you’ll like that it features a battery reading on the screen, so you’ll always know when it’s time to swap batteries. It comes with a lanyard included.