Zealots are deadly units throughout the game. They are the most powerful unit early in the game, able to take out 3 marines, or 3 zerglings. It takes at least 3 hydralisks to take out 1 zealot as well. They do possess a powerful melee attack, making them able to destroy anything that doens't fly very fast. Also They are small units, with 80 HP and 80 shields, which makes them very durable.
The first upgrade that you should get is the speed upgrade for these units. Since they move faster, it will make it easier to take out range units. The also make good scouts as well, since they are now able to cover the map effectively. The next upgrade should be focused on the zealots attack upgrade, which allows them to get a +2 bonus per upgrade. The armor and shield upgrades aren't necessary, unless you have a lot of money.
Zealots should be used to take an early advantage in a game, because no unit can match their offensive/defensive power. You should send your first zealot to scout, if your enemy hadn't been discovered yet by your probe. Also try to use them to contain an enemy early on.
Economically the zealots are very good units, costing only minerals. They are very powerful on large maps, where they can surround another army. They can be easily mass produced from gateways.
