For general aquarium testing the recommeded kits are:
Ammonia, Nitrite, and Nitrate
For planted aquariums these tests are recommeded:
KH,
PH, Nitrate, and Phosphate
Ammonia, Nitrite, and Nitrate are helpful for cycling and determining if there is a problem in the tank.
KH and
PH are necessary when injecting
CO2 to determine the level of
CO2 saturation acheived. Nitrate and Phosphate are helpful in determining correct fert dosing levels and tracking down issues surrounding Algae breakouts.
Most Iron tests are not going to be accurate enough for our uses from what I've heard from others on the forum. Perhaps if you're willing to spend an arm and a leg for a high end kit, but you're probably better off judging by the health of your plants. Same goes for Potassium and Calcium.
As far as brand, I really like the Hagen Kit. It comes with all the necessary kits and I find it very easy to read.