[douglas]

For just destroying all ships and units in a system, star destroyers, nebula creators, and black hole creators are all equally effective. A star destroyer gets rid of all stars in the system, converts all planets in the system to asteroids, and destroys all ships and units in the system. A nebula creator does the same and adds a nebula to the system. A black hole creator completely removes everything in the system except the warp points and adds a black hole - not even asteroid fields are left of the planets. In all cases the ship that causes the event is destroyed no matter what.

Tips on using these weapons of mass destruction: all of them require that the ship be at a star when you issue the destruction order. Star destroyers require that you be at the target star when you issue the order. Nebula creators can be told to move to another star and then explode. Black hole creators can be told to move elsewhere and then do their thing, and require only that the star they started at still exist when they finish; said star will be removed regardless of where it is in relation to the ship with no direct effect on other objects in that system.

Edit: Nebula creators are the cheapest to build, but don't forget that once it goes off you'll need some kind of cloak detection to see anything in the system.

Edit2: Nebula creators also require an extra step in rebuilding the system. You have to get rid of the nebula before you can create a star and start converting the asteroids back to planets.