Why are nonpolar covalent bonds the strongest?
In non polar covalent bonds the electrons are shared equally between the two atoms involved in the bond making the bond strong.
In Ionic bonding the electrons are shared very unequally. The electro negativity difference is large enough that the electrons are mainly around one of the atoms while the other has very little of the electron presence. It is mainly the attraction of a positive ion towards a negative ion that holds the atoms together.
Ionic bonds are easily broken as seen in the dissolving of salt (NaCl an ionic compound) in water.
In Polar covalent molecules the sharing of the electrons are unequal which makes them easier to break apart. An example is water.
In nonpolar covalent molecules the electrons are shared completely equally.