I see a lot of nuke reccomendations here, and I've tried a lot, but nothing seems to compete with bolter drones and a full complement of projectile cannons, with one ion cannon.
Ion cannons with fire rate and range upgraded almost match the range of the projectile cannons, and the projectiles are both a highly effective point defense, in numbers. Add the bolter drones to the equation with manual targeting, and you can tear nearly any threat to shreds. The drones are also an effective missile sponge and point defense in their own right.
