I'll pretty much echo what RR said. I use Salifert, Seachem are "okay", RedSea are horrid.
I have Nitrate, Calcium, Alkalinity, Phosphate, Magnesium, Strontium, Silicate, and Iodine.
I have a pH monitor (Pinpoint), and a Salinity refractometer.
I'd say you need
- Nitrate, Alk
, Cal, & Phosphate. Magnesium is also good to have. Strontium and Iodine are next-to-impossible to test for, and the kits are very frustrating. Silicate - only if you are having diatom issues.
You'll definitely need something to test pH and salinity.
You'll also need an ammonia and nitrite kit if you haven't yet gone through a cycle.