The API Freshwater Master Test Kit is the gold standard for checking the water chemistry of your aquarium.
But it comes in a really poor quality container that is impossible to use regularly. Using this design and a syringe, I am able to keep test time to under 2 minutes.
There are a couple different designs:
Master Kit: stores 8 test liquid bottles and 4 test tubes with the typical labels
Master + Hardness Kit: 12 test bottles and 6 test tubes