The bonds with the strongest or greatest ionic character are the ones created between alkalines and halogens. This is especially true of the halogens fluorine and chlorine.
Ionic bonds with the greatest strength are also called polar covalent bonds. The higher an element's electronegativity, the stronger and more likely it is to create one of these ionic bonds. Florine, for example, has an electronegativity rating of 4. An element's electronegativity goes up from left to right along the periodic table, so in order to make a bond with the greatest ionic bond, use as many elements from the right hand side of the table as possible.