[michika]

I would suggest:

Nitrate, Ammonia, pH, and Phosphate (this one is iffy), refractometer for general keeping. I think I use my nitrite kit maybe once on average before it expires, and I only use it when I'm starting up a new system.
Magnesium, Calcium, and Alkalinity for SPS and LPS.
As bonus add ons, I like ORP, but it isn't a must-have in my mind.

As for brands, Elos is a nice kit to use, simple, and mostly straight forwards, Salifert is good too, but I find some kits hard to replace/find. I also use PinPoint monitors (Nitrate) and Hanna's Phosmeter (phosphates).

I don't know what the fresh/saltwater box kit is sorry...