Salifert, LaMotte, Hach, and Elos -- all high quality, typically titration test kits -- about as accurate as a hobbyist can hope to get with out lab analysis equipment. API tests aren't the worst, but they definitely have a bit more tendency to fluctuate between readings. Bills recommendations are all spot on, IMO -- verifying your test results against another test kit (and another tester, especially if you're still getting the hang of the testing thing) is a good thing. Helps verify the effaciacy of your tests, as well as the accuracy of your methods.

-Justin