Laser Turrets.
Also they help you clean out the rest of the system. 
A single mark 1 laser turret does 91500 damage to a fortress, a bomber starship does 93500, bombers Mk 1 do 12000.
If 8 mark I bombers out damage a mark I laser turret I'm going with mark I or higher bombers. However laser turrets fire every 4 seconds while bombers fire every 12 seconds & the bomber starship fires every 2 seconds. Therefore your values in DPS terms are:
Mark I Laser Turret - 22875 DPS
Mark I Bomber Starship - 46750 DPS
Mark I Bomber - 1000 DPS
Where do you get the 91500 figure from? A mark I Laser Turret does 1200*3 damage & has an 8* multiplier against ultra-heavy hulls. That works out at 28800 damage per volley or 7200 DPS which is still out damaged by 8 mark I Bombers. Also Bomber Starships do 93000 damage per volley.
However you're not taking the 2500 armour of the Fortress into account. As the Laser Turret has 1000 armour piercing that's effectively 1500 armour vs them which changes the values to:
Mark I Laser Turret : 24300 volley = 6075 DPS
Mark I Bomber Starship : 90500 volley = 45250 DPS
Mark I Bomber : 9500 volley = 791 DPS
Mark I Laser Turret is still out damaged by 8 mark I Bombers.
Also of consideration is the Raid Starship. The mark I does 11200 damage with a 2* multiplier against ultra-heavy with a 3 second reload & 100k armour piercing. That works out to 7466 DPS. Seeing as it has double the health, 18 times the armour, 100 times more armour piercing, immune to the Fortress missiles & moves like greased lightning it's a far better choice.
I'm using 4.032 so 4.021 may have different stats for the Laser Turret but the release notes only say the HP & range of the turret has been changed, nothing about damage, reload times or attack multipliers.
