I also use The Aquarium Pharmaceuticals Inc. test kit. When I first started keeping fish, I used test strips, but they aren't very reliable and kept giving me wrong readings, so whatever you do, don't buy test strips, they are innacurate. Like Carol says the API test kits are very reliable and are easy to use.
Same here: I use Aquarium Pharmaceuticals tests. I think they're very accurate and never had a problem with this test kit. I think it's the best one on the market. It can get expensive at local fish stores, but it's very cheap on Big Al's (the link that Rose gave you). If you want a good and reliable test kit, definitely go for Aquarium Pharmaceuticals.