Well, best is halon, as it is non corrosive, evaporates off and has no static problems. Does no further damage than the fire.
Next is CO2, no damage to equipment, but has a bad static generating potential and can actually ignite flammable gases. Best for your electronics as it is the only one available off the shelf, and with a good shelf life. Note, you do want to get a 2kg unit with an aluminium housing, it is a lot lighter to move around and mount.
Dry powder is best if you only want to put out the fire, as it is somewhat corrosive, goes everywhere and is difficult to clean out of equipment, you will probably scrap it afterwards. Good for fires involving flammable liquids and kitchens, as it will handle most fires there, with a powder that smothers most fires and is low enough velocity not to blow burning liquid all over.
Water is cheap, plentiful and will put out most fires eventually. Bad around electrics, electronics, but very good for wood, paper and building structural fires.Messy, and the runoff can carry burning liquids.
The most commonly available is dry powder, cheap and easy to store. Next is CO2, and then water.