The big 4: Calcium, pH, Alk, Magnesium.
You can do nitrate if you want. Its tough to find a *good* phosphate kit. Just look at your algae in the tank to measure phos.
Strontium, completely useless test IMO. Nobody REALLY knows if its really that important for coral growth/health. If you do regular water changes, you're probably getting all you need. I'd say only test that if you're dosing.