Api master kit is the cheapest most readily available. However, if you want really accurate measurements, and high quality test kits you should look up salifert, and red Sea, or Hannah.
Of course there's the matter of what to check. Dose only what you can test for, use a High quality buffer and a good calcium additive. So, refratometer or hydrometer, alkalinity test, magnesium, and calcium test kits. Magnesium you might not even need. Of course ammonia, no3
and no4, phosphate those are your default ones.
Strontium, barium, iodine, iron, silicate, oxygen, orp.... Well they're for people with more free time than I.