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It depends on your needs. For fish only, API calcium, nitrate, alkalinity, will do pretty well. For specific gravity, a refractometer is best.
If you want to get into having hard corals, you should get more refined tests. Hard corals grow by enlarging their skeletons. The use alkalinity and calcium to make calcium carbonate skeletons. Also magnesium levels become important.
SPS corals are more demanding than LPS corals.
For them, most people measure alkalinity daily and dose to keep it constant. I use Red Sea Pro for ALK. Many people use Salifert with good results.
Calcium is also important. Sea water contains a bunch of calcium. I measure weekly with Red Sea Pro but Salifert is good also.
Magnesium is important. The Red Sea Mg test drives me crazy. I use Salifert. I have found that routine water changes with a high Mg salt mix keeps it at good levels so I rarely test it.
SPS corals are sensitive to high nutrient levels but you want some nutrients or they will die. I use Red Sea Pro for nitrate and I like it better than Salifert.
I use Hanna ULR phoshaphate tester for phosphate. It is the only hobby test that is sensitive enough on phosphate to be useful.
Of course, when you cycle a tank, you will want an ammonia test and a nitrite test. Ammonia is quite toxic at low levels. Many people report that the API test gives a low reading for ammonia when it is actually zero. I guess they are fine with that. It would drive me crazy and I use either Salifert or Red Sea. Once, your tank is cycled, you will rarely have any need to test ammonia or nitrite.
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