Salifert is recommended.
In the begings of a tank you will want:
Ammonia
Nitrite
Nitrate test kits
If a reef you will also want:
Calcium
Alkalinity
Magnesium (used only once a month, more if problmes maintaining).
After your tank matures you might not need the ammonia and nitrites very often if at all. Only when something strange happens or something dies in your tank will you need them again.
The calcium and alkalinity is a constant testing for me, once every few days or particulary before adding a new fish or before a water change and then again a day after a water change will I test for alk and cal.
Those are the main ones that I would recommend
Thomas