For aquarium purposes, I don't think accuracy is the most important thing. I'm content with test kits whose results are more or less accurate, but I think it's imperative that the results are reliable and that testing is easy.
For example, I'm not terribly concerned if my pH is 7.8 and the test kit reads 7.6. I do care that the test repeatedly measures the same way (i.e., it's reliable) and I do care that it's easy for me to test. Similarly, I don't care if my tests can accurately detect .20 ppm nitrite vs. .25 ppm nitrite; I do care if it's clear if there is ANY nitrite and if it can show me the difference between .25 and 1 ppm.
If you're doing research and want high accuracy, or if you have extremely sensitive fish, or if you're just starting out and want to learn about water chemistry, accuracy may be more important. For many of us, being able test quickly & reliably is more of an issue. I like dip stick tests for this kind of quick and dirty testing. For more precise testing, I like the kinds of tests where you look thru the treated sample of water and compare its color to a translucent piece of plastic with graded levels of color. I find these much easier to read than the kind where you compare the treated water sample to a printed color card.
Of course, electronic probes are probably the easiest and most accurate, but not cheap...
HTH,
Jim