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 :)