You mean battleship/cruiser. Star Destroyers are definitely not designed for ship-to-ship combat from what I've seen in the movies and games. They have limited missile capacity and in order to get into turbolaser range they're putting themselves within firing range too. Their main starfighter complement, as well: TIE Fighters. Whoopdedoo.
Mon Calamari cruisers and Frigates both have more missile batteries than a damn ISD. To make matters worse, they carry better starfighters, all of which can be armed with torpedoes or better. The main plus of an ISD is the fact that it has a lot of flat surface area, so orbital bombardments take a lot less time.
In the EU and games, at the very least, the Empire's
foremost anti-starship weapons are the Assault Gunboat and the Missile Boat. Both of them can operate independently from a Star Destroyer.
The Empire doesn't build
effective. They build big, and they build lots. I don't think it's hard to see why the Imperial Doctrine calls for attacking enemies with a force three times superior.
Also, according to the knuckledraggers on SD.net, because there's a "Star Destroyer" there has to also be a "Star Battleship" and a "Star Carrier" and a "Star Frigate". They are so goddamn stupid.