Joe, there is no "best" machine. One man's favourite will be hated by another, and vice versa.
Take the parts out from the plastic casing and you have some vacuum motors and a pump.
To clean carpets and upholstery effectively and without taking all day, a machine NEEDS:
> At least two 3-stage vacuum motors
> At least 100psi pump pressure, preferably more (but beware of some big pumps in portables because of reliability issues.)
> At least 30 litres capacity in both solution and recovery tanks
A machine is a very personal thing
Some of the main contenders are (in no order of best/worst):
Ashby's Ninja
Solution UK's Scorpion
Mytee Speedster
Alltec (several options)
Prochem (several options)
Powrflite
Extracta
Your question is a bit like asking "What is the best car?" Imagine trying to get Clarkson to agree with Hammond on that, and you'll be close to the other million threads on forums between carpet cleaners arguing about which machine is best.