The 4th finger on the left hand, the finger next to the pinky:
The long answer is that there is no right answer.
The historical custom for wearing an engagement ring is thousands of years old. Hard evidence proves the exist since the days of the ancient roman empire). It is also believed that the custom began even earlier with the Egyptian dynasty.
The reason of wearing an engagement ring on the 4th finger of the left hand is the old superstition the there is a vein connecting the heart to this finger.
Important to know!
In the western world it is costumed to wear the engagement ring on the 4th digit of the left hand.
For most humans the fingers on the right hand have a bigger size than the fingers of the left hand so the sizing is done for a specific hand.