I think it depends on what your testing as to what the best test kit is....If it's for phosphate, I personally think the only choice is the hanna ULR phosphorus test (just makes sure you understand you need to take your result * 3.066 and then divide by 1000 to get the result for phosphate).
In general I have been VERY happy with the red sea pro tests kits. They have personally worked the best for me for getting a visual result that I can identify, as well as being able to reproduce results.