which bond is more strong IONIC BOND or COVALENT BOND.and why?
First of all, it bears mentioning that there is no line between ionic bonds and covalent bonds in nature. This is a line that scientists draw. Rather than being a logical (either it's covalent or it's not) think of it as more like a specturm, with some bonds being more covalent than others, and then having more covalent character than others.
Second of all, it depends A LOT on the kind of atoms your using. Big ones with high atomic numbers have trouble getting enthusiastic about much of anything because they have so much shielding going on from the dozens of electrons they have. So no rule you make is going to hold entirely true - some ionic bonds are stronger than some covalent bonds, and some covalent bonds are stronger than some ionic bonds.
Having said all those disclaimers, the answer is that more ionic character generally makes a bond stronger. Here is why: Just think about what would happen if you were to break the bond by pulling the atoms physically apart and then re-establish it by shoving them together again. If you had a strongly ionic bond, like H-F, when you separated them hydrogen wouldn't have a chance of keeping the shared electrons when faced with flourine's massive electronegativity. You'd end up with H+ and F-. On the other hand, if you pulled H-H apart, their completely equal electronegativities would probably result in each of them getting one of the shared bond electrons, turning them into two H radicals with a single electron.
Here's where the difference in important. An H radical is electrostatically okay. Which is to say that chargewise, all the electrons and protons are 'happy'. Of course, a free electron would much rather have a partner, so it's probably not going to stick around like that... but compare it to the alternative. F- and H+ are both electrically charged. Even if we pull them apart, the electric force is going to try and suck them back together again. They are NOT electrostatically okay. And the electric force wicked strong (it's 10^36 times stronger than gravity!).