One of the best investments I have made in the last couple of years has to be purchasing the Hanna test kits. I don't test compulsively, but my system is operating at maximum coral load, so keeping an eye on things seemed prudent. I bought almost all of them except the Ph kit, my reef controller monitors that fine.
Ultra low nitrite (ppb)
The phosphate tester is a must have if you're trying to raise corals IMO
as it gives consistent measurements below .1 ppm
Alkalinity, again a must have if you have a heavily loaded tank, alk
can go south on you pretty fast.
Calcium, it's a nice tester, but maybe not as necessary as the others as you don't worry much about overdosing calcium and good reef salt is loaded with it already.
Ultra low nitrite. This is a cool one. Thought you had zero nitrites? Not so much, as this measures in ppb. Mine shows 18 ppb, which is really low. That's .018 ppm
Iodine. Got sick of not knowing and looking at the iodine supplement bottle thinking if only I would add some, my SPS
corals would double in size over night. Did the test and it was high at .1 (natural seawater is less than half that). So at least it retired the iodine bottle.
Bulk Reef Supply sells them for about $50 each. There are reagents that come with the kits, but you will end up ordering more. Color charts work, but this is much more consistent...and easier.