Put simply, the best position is the one that feels the most comfortable at the time, and the one that allows your baby to be born. There are many positions for birthing: all fours, kneeling, squatting, standing and side-lying. I haven't mentioned the on-your-back-on-the-bed position because this is seldom chosen by women when they are given other options and are free to choose between many positions.
Things to consider with birth positions are comfort, ease of pushing or breathing the baby out and the likelihood of tearing.
The most comfortable positions are usually kneeling and all fours. In these positions, tearing is least likely and the birth is likely to proceed well because your pelvis is able to open fully.
Incidentally, the on-your-back-on-the-bed position is likely to result in tears, more effort required with pushing and a longer second stage.
In my practice, all fours seems to be the most common position that women choose, however I was fortunate to be present at two squatting births recently.