There are pros and cons to all test kits. I've used Hagen, Salifert, Seachem, API, and Elos and I honestly cant recommend one as 'the best'.
If you're just cycling a tank right now a Nitrite and Nitrate kit would be good.
For keeping stoney coral later I like Calcium, Alkalinity, and Magnesium.
Phosphate is nice to have too but generally you can just look at your tank and if you see a lot of problem algae you will know you have high phosphates (but at least with a kit you'll know how high).
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