I’ve used a variety of test kits over the years.
For nitrate and nitrite, I struggle to read the liquid-based test kit results, irrespective of brand; I’m currently using Waterlife but plan to try JBL next time as I find their products very easy to use. I always cross-check nitrate and nitrite liquid-based tests with test strips (either Tetra or JBL) as I find them much easier to read.
For ammonia/ammonium, I use JBL which I’m happy with – and it’s possible to buy refills rather than an entire new kit. I also bought API ammonia strips but find the liquid-based JBL kit easier to read.
For GH and KH, I currently use Tetra and API respectively, although any of the brands I’ve tried before have been fine / easy to use and decipher. A useful tip, which I picked up from Sue, if your reading is borderline/difficult to decipher, is to test 10ml rather than 5ml, then half the overall number of drops required. For example, for my low KH, I find that 3 drops are usually required in the 10ml to produce the colour change, equating to 1.5 drops for 5ml and therefore a KH of 1.5. I find that the test strips (both the aforementioned brands) give an inflated GH reading while the KH reading is difficult to decipher but seems to be inflated too, in comparison with what I would expect from my water company details.
For PH, I currently use JBL. I tend to find any of the liquid-based kits err slightly on the inflated side, while the test strips err on the lower side of what I might expect from my water company details.
The test strips have the added advantage of a chlorine test on them.
Hope that’s helpful.