Most important:Temperature and salinity.
If you have corals, add: Alkalinity, calcium, magnesium, nitrAte, phosphate. You can add in a bunch of other things as you progress, but those will cover pretty much everything you'll worry about if you are doing regular water changes. Folks that try to minimize water changes often use ICP tests for a much broader spectrum of elements.
Temperature can vary a bit, but you are going to want to keep your salinity stable. That will help you to keep all of the other parameters stable.