Standard is aluminum. It is inexpensive, durable, resistant to corrosion, and available in a wide array of colors.
Galvanized Steel is stronger than aluminum making it resistant to dents, but can be prone to rusting. Steel gutters are also stronger than aluminum, but is more resistant to rusting than galvanized steel, which makes it more expensive.
Personally, we love copper gutters because of their beauty as well as durability. Most standard gutters come in the standard "ogee K-style" with a flat bottom, or you can go with a more stylish 1/2 round rain gutter. The latter look especially classy in a copper material. 5" or 6" are available.