In general I use rolled oats, also known as old-fashioned oats. The difference between rolled oats, steel-cut oats and instant oatmeal is how the oat graots are processed. Oat groats are the inner kernel of the grain. First the outer hull is removed exposing the groat, then the groats are steamed which gives them a longer shelf life (otherwise their natural enzymes make them go rancid). Groats are then either rolled into old-fashioned flakes or chopped into small pieces called steel-cut oats. Instant oatmeal is an extremely thin form of oat flakes. Because they are the most processed, they have a higher glycemic index value meaning that their starch is converted into sugar and released in the bloodstream more quickly. Most instant oatmeal is highly sweetend and laden with artificial flavoring and additives, making it a sweet treat rather than a healthy, satisfying breakfast.
Oats have extensive nutrional benefits, in particular the benefits of oats on blood sugar and cholesterol are well known.
For more about the health benefits of oats: Health Studies on Oats