For the deepest restorative clean we lean toward the Rotovac powerwand.  We never use the standard wand unless we are cleaning stairs. This method combined with pre-shampooing and followed with the VLM method cannot be beaten when seeking the absolute cleanest a carpet's surface can be.
The method we prefer the most is a Very Low Moisture method which cleans, traps dirt and the residue in a crystalline polymer for vacuum removal later on.  The carpet is dry in an hour rather than four to eight hours. Both methods utilize GREEN chemicals to lessen our impact on the environment.
Think the Rug Doctor is a good solution?  Think Again!!
Bringing together all the power we can muster to make your carpet whistle clean