[Reefiez]

My own preference:

Calcium: salifert / red sea
Alkalinity: salifert
Magnesium: salifert
Nitrates: salifert + red sea (i usually take an average), API for rare cases where nitrates are insanely high
Posphates: hannah checker (ppb), really takes out the human eye error
pH: API high range (don't run a probe)
Salinity: Hands down the milwaukee refractometer, which weirdly happens to be my most satisfied purchase in the hobby

I'd also recommend a TDS meter of some sort to make sure your RO/DI water is good, if you don't already have one.

If you are dosing these elements, i'd also testing for:
Potassium
Iodine
Iron