Generally, the closer the clamping voltage is to 330 volts, the better. Try to find a model with a Joule rating of 300 joules or higher. The joule rating is an indicator of how much energy the suppressor can handle before failing. Therefore, it is an indicator of how long a life expectancy the unit may have. Unfortunately, not all manufacturers calculate the joule rating the same way.
Different calculations can result in widely different joule ratings.
It is probably best to get a suppressor with a Maximum Surge Current rating of 10,000A or higher. This is the amperage rating for the device. There is also a Maximum Surge Voltage rating which will almost always be 6,000V, but then even the regular outlets in your home wiring system will arc over at 6,000 volts. So, maximum surge voltage rating is not very important. Maximum Surge Current ratings are important and can vary greatly.