100 battle cruisers
the key is to compliment your main gun's weeknesses. Battlecruisers dont hit hard but they can take a beating with their high shields and hit points, and have rapidfire against A LOT of ships so they need fodder. Destroyers pack a whallop but they dont have rapidfire against fodder and the most comon ships (cruisers, battleships) and they cost a lot to move. so try to balance out your fleet's weaknesses.