Too often, watering cans look bright in the shop but have no other virtues. They should be:
Decoratively, watering cans can sit like cats in gardens - always in well chose positions and always adding to the scene.
Functionally, a watering can needs:
The spout should be slightly higher than the height of the body of the watering to allow the watering can to be filled to the brim. Roses can be either round or oval. The shape and angle of the rose will determine the force of the stream of water from the rose.
As to materials:
It is worth considering buying two watering cans - not only will this mean that you can transport more water but it is also more ergonomic as you are balanced when you have a can in each hand. It is traditional to have one red watering can for soluble fertilizers and one green watering can for pure water.
Galvanized iron watering can
Rusting watering can
Plastic watering can with hoop handle
Watering Can, Chelsea Flower Show 2008