I don't really think there's a "one brand fits all" test kit. I like Salifert for some tests (nitrates, phosphates, calcium), but like Aquarium Pharmaceuticals for others (pH, alkalinity, ammonia/nitrite) - it all depends on what you're looking for.
Some test kits will test with higher sensitivities than others, so that's what you need to look for. For example, if you use the API test kit for Nitrates, the first color change happens at 10.0 ppm
. So until you get to 10ppm, you're just guessing at what your levels are. With the Salifert Nitrate test kit, the first color change on the card comes at 0.2ppm when you're using the "high sensitivity" instructions. So you can see where some brands would be better than other... assuming you're looking for very low levels of nitrates.
Regarding your comment...
...but it doesn't have tests for magnesium, iodine, moly, all the other elements I was advised to add.
Assuming you're doing weekly, or even biweekly water changes, I would doubt you need to add ANY "other elements" to your tank. Unless you have a ton of corals, supplementation of anything other than a basic alkallinity/calcium supplement is unnecessary in my opinion. And depending on your salt mix, the alk
supplement might not even be needed. Sounds like you have a LFS
trying to sell you a bunch of additives you don't really need.