I designed my fleet while playing through the highest difficulties, and completing the Impossible difficulty with the Long Run setting active.
They are simple, with very little unnecessary aesthetics, and meant to be efficient in terms of energy and machine numbers.

Energy Carriers T1-3
Transporters Solid T1-3
Transporters Liquid T1-3
Asteroid Miner

Constructors:
Construct the first one with a construction cannon. They are made from their own material so that they can repair themselves.
For Iron, I like to have two. For Warfare, you'll want two "repair" ships.

Combat Ship 1:
Adjust this to your tech. Use Railguns or Missiles of any kind. Do not use with lasers - will run out of ammo.
Titan Gold Composite is default and meant for late game, when Ancients start attacking you - four of these with Nukes will easily fend off their early attacks

Combat Ship 2:
Construct from Titan Copper or Titan Gold. Living armor if possible. Can be constructed from captured T5 ships from green faction.
For living armor: construct from the "seed" blueprint, assign the full blueprint, then let it "grow" to save materials. Small amounts of living composites can be created with the Matter Materializer.
Adjust the storage to your tech. Lasers below green are not better than nukes.