Salinity with a refractometer.
Phosphate and nitrate I generally don't bother with.
I also like the Salifert for Mg. I used to make kind of a homemade version with a HACH kit (super cheap per test), but I test for Mg so rarely it's not really worth it as the reagents would get old.
I really like the Hanna kit for Ca. But you of course need good lab technique. I also don't test very often for Ca. Maybe once every few months.
For alkalinity (which I test for about every 3 days or so) I've made my own reagent using Bromophenol blue and some sodium hydroxide and HCl.
Here's the receipe. You can make gallons for very low $.
Alkalinity test kit reagent recipe:
0.2 g Bromophenol blue (Sigma sells it, other chemical supply companies also, might be able to get it online somewhere)
0.2 ml of 5 Molar NaOH to help dissolve the Bromophenol blue
800 ml DI water
4.15 ml concentrated HCl (lab grade HCl is 36%, you could also use muriatic acid from the hardware store, but you’d have to use a bit more)
Use 15 ml of tank water and add the reagent dropwise. The color will initially be blue, then change to a yellowish/orangish tint when the titration is complete. Depending on the drop size you get out of your dropper bottle I usually find one drop equals 0.2 meq/l. I usually add it from a 1 mL syringe. You of course can test your reagent against a standard test kit like the Fastest kits.