I would say water; water is extremely polar, and this is why it can break one of the strongest bonds, ionic bonds. NaCl, as you probably know, is a salt, and dissolves in water. However, the ionic bond holding the Na+ and the Cl- is extremely strong; the boiling point of NaCl is at 1413 degrees celcius (water is at 100 degrees celcius). This means that it requires A LOT of energy to break the bond, but water is able to dissolve and break the bond very easily. It is very polar, so I would answer your question with water. And the bond connecting the H and the O is a covalent bond.