The position you should labor and give birth in is the position that is most comfortable for you and which uses gravity to help pull the baby down the birth canal.  If you are listening to your body and spirit, you will automatically feel the need to change position when it is the right time to do so.  The best positions are those which keep you pretty much upright and able to move around as you feel the need.  The worst positions are those in which you are lying down.  If you must lie down to rest, lying on your left side (with your right leg elevated if you are in the delivery stage) is best, but you should not lie down for a long period of time.  Your contractions will be more effective, less uncomfortable, and your labor shorter than if you were lying down.  Here is a summary of those positions most often used:

Upright Positions

Standing
Standing Supported Squat - supported by partner or wall
Kneeling
Supported Kneeling - supported by birthing ball, bean bag chairs, couch, bed
     low table, or pile of pillows
Squatting
Supported Squatting - supported by sitting partner, wall, squat bar, or hanging
     suspended rope

Semi-Upright

Hands and Knees
Semi-reclining

Non-Upright

Side-lying
Supine of lithotomy position

I do not suggest you ever lie on your back after 34 weeks gestation, because this causes the uterus to depress some major blood vessels, impairing oxygen to the baby.  I also do not suggest you labor or birth in any position which depresses the tailbone, which can  obstruct the birth passage.  Squatting pulls the tailbone out of the way.  Even the semi-reclining position causes some obstruction for some people.

 Benefits of The Squatting Position