If you're referring to a water testing kit to test your water's pH, ammonia, nitrite, and nitrate, I'd recommend the
API Freshwater Master Test Kit. It is usually around $17-25 depending on where you get it from, and its test results are much more accurate than the little paper strip test kits that are available. If you follow the link I provided, you can get it from Big Al's for $17.99 before shipping.
If you're looking for kits to test for
GH/
KH or phosphate, API also makes decent kits for those as well which are sold separately from their master test kit.