It is proven to be the safest for visibility although it's often not popular with motorists who are keen to overtake you.
It takes confidence to cycle in the middle of the lane and you can become subject to sounding horns, revving engines, verbal abuse and an array of hand gestures, but rest assured you really should be there for the safety of all road users with optimum visibility.
Image courtesy of British Cycling.
For further information on Prime Road Cycling Position visit;
http://www.britishcycling.org.uk/cycletraining/article/ct20110110-cycletraining-Bitesize-Bikeability--Part-4--On-Road-Positioning-0
