I have a friend who is a professional carpet cleaner. When he decided to start his business he did a lot of research to find out which were the best machines and what was the best way to clean carpets. Of course, there is no exact answer. Different people like different machines for a variety of reasons.
Eventually, my friend settled on a machine that used foam to clean carpet. The foam gets the carpet damp, but doesn't leave the carpet as wet as the hot water extraction and some of the other methods of cleaning. Usually, the carpet is dry enough to walk on in a few hours without any worries.