There seems to be widely accepted consensus about this, it's index and middle fingers, never ring finger. Thumb is now accepted for plucking too, ten years ago it was only for slapping. I started to play the bass (45 years ago) using index, middle and ring fingers. Later I involved my pinky and thumb too. My middle finger is much longer than index and ring which are much closer in lenght. If I must play quick triplets, 16th notes or something like that, I tend to use alternating index and ring. I remember reading (one of my great heroes) Nate Watts discovered the power of his ring finger after having an accident with his middle finger and not been able to use it for a while. Your thoughts?