I'm sure that you people all know that Imperials are very bad shooters in TIE Fighters, right? And in the space battle in KOTOR 1 at the end shows that Sith Fighters are much more skilled than the Imperials. I mean... the Imperials couldn't even hit the Millenium Falcon unless they're lucky, and Falcon's big compared to regular fighters.
And I'm sure you can judge two entire Empires' filled of pilots by seconds of cinematics, right? Keep in mind the Empire was ordered to keep the Rebels alive for a show for Luke. Not only that, the Empire has far greater numbers and far greater ships. Pilots are a variable, since there are tons of great Empire pilots in the EU.
But Imperial Star Destroyers have tiny turrets all lined up on their surface, but Sith Flagships have 20 lasercannons (seen in the bombardment of Taris, Knights of the Old Republic) all on one side, and could simply bombard Star Destroyers from the sides or above.
No. A single ISD can take over a dozen of Leviathan. Leviathan = 4 turbolasers, ISD = 60 turbolasers. And the Leviathan = flagship. The Executor or the Eclipse could probably take on dozens of Leviathans.
And their lasers do puny to Taris. They really couldn't hurt an ISD unless they use their small amount of Turbolasers.
And yes. I know that Imperial Star Destroyers are huge compared to Sith Flagships. (Star Destroyer: 1200 metres, Sith Flagship: 600 metres: From info from Starwars.com), but it's not size that matters. I mean you all remember Hannibal right? Always remember that he defeated Roman armies much bigger than his. It's not numbers that win battles: Its the strategies of the leaders that wins battles.
1600 metres, actually. A flagship being nearly 10 tens that. And 25,000 of them too.
Plus, Thrawn is the greatest military tactician the galaxy's ever seen... the Empire wipes its ass with Revan.