Well, I'm a little bit prejudiced on that point, since I sell 3 different CCPWMs from my online stores.
For a basic one, get this one: http://www.fuelsaver-mpg.com/accessories/40a-pwm-basic
. You'll need your own amp gauge with this, but it will do the job up to about 25 amps continuous usage.
If you would like some more functions, and display with readouts in the passenger compartment, get this one: http://www.fuelsaver-mpg.com/accessories...controller
Finally, if you have a large engine or large cell, you can get our heavy duty version here: http://www.fuelsaver-mpg.com/accessories...th-display
. It has the same display as the one above, but can be used on very large hho systems such as would be used for semis, large gensets or any large engine up to about 50 liters.