For Actual defense, Quick reaction forces do well, as said above.
Against enemy missile strikes, small raiding forces, etc, a PDC with AMMs might be a good choice, it defends you from missiles while your atmosphere protects you from beams.
A little short range punch versus Mesons is never a bad choice, though.
For keeping the populace at peace, look at the PPV, Planetary Protection Value.
More and bigger weapons is good, so a PDC with some huge honky Missile Launchers helps keeping the peace.
The other way to go would be to suppress riots with Ground forces, Garrisons are cheap and do the job. For real defensive power against ground attacks, though, I would prefer mobile Infantry Divisions housed in PDC Bunkers.
It always depends on what you want to defend, and if it actually needs to.