In the only scientific testing I've seen all the test kits normally used by hobbyists were, frankly, rubbish. The drops were no more accurate than strips in picking up trends, though it is near impossible to get 'numbers' from strips, and near impossible to get accurate numbers form most drops until you're spending 70 dollars plus per single test kit.
So, FWIW, I use tetra and hagen pH and kH kits, salifert for ammonia nad nitrate and a Merck/Deltec phosphate kit and a salifert copper kit for emergencies. By their very nature pH and kH are the most reliable kits.
So buyer beware!