I used to use Salifert but I am reading good things about API. I'm a bit nervous to spend the money on the digital readers after hearing some really negative reviews.
I use:
Red Sea for Ca, Mg, No3, K, and Iodine (hardly ever use K or Iodine)
Hanna checker for phosphate and alkalinity
I'm red green colorblind and I found the Red Sea were the easiest for me to see the color change. I d never had any issue with the digital checkers, except for the Ca checker. The Ca checker uses .5 ml of tank water for test which IMO is too small. Too much variation in the results.
That said I've seen great tanks whose keepers have used ATI. This is just what worked for me
I think it depends on what your testing as to what the best test kit is....If it's for phosphate, I personally think the only choice is the hanna ULR phosphorus test (just makes sure you understand you need to take your result * 3.066 and then divide by 1000 to get the result for phosphate).
In general I have been VERY happy with the red sea pro tests kits. They have personally worked the best for me for getting a visual result that I can identify, as well as being able to reproduce results.