We learned this from Alton Brown: the fatter the garlic clove, the more mellow the taste.
Wait. Are you all set on what a garlic clove is, exactly? This confused us too, so allow us to explain it.
A head of garlic is the whole thing, many cloves covered by papery skin, with a root end and a long papery top.
You generally see fresh garlic in this form, or braided into a rope. It is picturesque and, for cooks who are more garlic-knowing than I, it might produce better results.
Sometimes I will use a whole head of garlic cut horizontally, like for a soup or stew which is going to cook for hours and then be strained, but for the most part I use prepared cloves of garlic, and I recommend them to you as well.
In most markets these can be found in plastic cup or pint containers with the store's own name on the label. In other cases the cloves are in a jar or a bottle with a manufacturer's name. Both are good, but do look inside and check to see that there is no white fluffy mold growing on the cloves. (It can happen).
When you buy your garlic pre-peeled, it becomes more a vegetable than a spice. In other words, you use more of it. And if you're going to use more, you're probably going to want each clove to be a bit milder then the average, sharp-tasting garlic.
Therefore, look for the fat ones.