Unless you are talking about yet to be invented explosives, at relativistic speeds you may as well launch a rock. The explosive power of your warhead will probably be irrelevant when compared to the kinetic energy of the projectile.

But, fairly enough, launching anything at relativistic speeds requires yet to be invented propulsion systems that will require energies much larger than any explosive we know today.

For maximum efficiency, you could launch a lump of antimatter at relativistic speeds. That should take care of the details.