I used to use the Hagen tests. Mostly because they are the most widely available here. What I liked about them the most - You could buy replacement regent bottles at Big Als when they ran out and reuse the test tube, etc., therefore saving money (if you were buying stuff from Big Als anyway).
Many people here recommended the Aquarium Pharmaceuticals Master Kit. When I needed replacements the next time, I ordered AP. I like them a lot. What I like about them most – User friendly. As long as you have your regent and the colour charts, you’re good to go. The colour charts have abbreviated symbol instructions on the back. The regent bottles all have the number of drops printed in a drop on the label. The company website also has instructions for use on a link for each product.
Compare that to the Tetra test. Now, all I have is the Tetra pH test, so I can’t speak for them all…. BUT I still had the colour chart and somehow lost the instructions. NOTHING on the regent bottle, nothing on the box, nothing on the back of the colour chart. The company website lists products, but with no more information that is on the box. No instructions. They do have a Tetra Care extension site which might have the info, but even though free, you have to register with your email and home address and everything. No thank you. It took a lot of google searching to find out I needed to add 7 drops.
#1 - TetraTest Laborette. Decent kit and relatively easy to use. PH tests are somewhat vague as they're in .5 increments so it's tough to get a precise reading.
#2 - Aquarium Pharmaceuticals Freshwater Master Test Kit. PH tests are far more accurate. Ammonia Tests are a bit odd though as it will show the "Ammonium(non toxic stuff) and doesn't distinguish it from the actual Ammonia(harmfull stuff) so even with treated water, reads .5 when there's nothing toxic.
I use AP and Red Sea. I prefer the AP over Red Sea any day, especially the KH/GH tests. They should be shot for those ones
If you run out of a particular test, like I just ran out of nitrates, you can order just the kit you need.
I also bought a box of 24 extra test tubes. Now I can test all my tanks on Friday and THEN wash all the test tubes. Heck of a lot faster than testing, washing, testing, washing, repeat 10 more times, blargh!