Depends on your goals.
If you are looking to just have low levels of ozone dissolved in water, the the OXS-300 is a simple and effective device. See link below:
http://www.oxidationtech.com/products/ozone-generators/ozx-300.html

If you have bottled, or oxygen concentrator, then use the 1KNT. It will produce higher levels of ozone, and higher concentrations of ozone, therefore, increase ozone level in water:
http://www.oxidationtech.com/products/ozone-generators/1knt.html

Not sure your goals or what you have.

If you are looking for basic ozone generator to bubble ozone in water see the OZX-300 below:
http://www.oxidationtech.com/products/ozone-generators/ozx-300.html

If you have oxygen available and desire something a little higher concentration, the 1KNT could work much better:
http://www.oxidationtech.com/products/ozone-generators/1knt.html