What kind of seeds are the healthiest as a diet supplement?

Many. If you're going to eat seeds, they should be organic, raw ones. Chia seeds come to mind, because they are high in omega-3 fatty acids. Most of us don't get nearly enough of these essential fatty acids in our diets. Also pumpkin, sunflower, hemp, and sesame seeds. Any of these can be eaten as food - you don't have to take a supplement capsule.