Executor-Class SSD

Lore:
This warship was created as nothing short of the ultimate symbol of Imperial power.
At 19 km in length, only the Vengeance-Class Dreadnought could approach it in terms of sheer scale: a monster even among SSDs, no ship at the time of it's construction being capable of destroying it with conventional weapons. The personnel required to man the ship meaning that noone other than the Empire had the manpower to field such a ship. Her cost in both resources and credits meaning many planets had their populations starved, impoverished and enslaved; resources and people being completely stripped from Imperial subject worlds to feed the construction of the Gargantuan vessel.
The ships armament was as unrivalled as her size, able to boast more fire-power than whole fleets of lesser warships combined.
She wouldn't be overtaken in the Size or Fire-power department until the Development of the Equally menacing Eclipse-Class SSD. Despite these ships' impressive size and fire-power, they where often denied the chance to put it to full effect due to the weak and fragile nature of much of her opposition: The Empire later (wisely) deciding resources where better spent on the much more economically viable Imperial-Class ISDs. Although her legacy lived on with her smaller, cheaper, cousin the Belator-Class SSD.
Armaments:
2000x Octuple Fire-linked Heavy Turbo Lasers
2000x Octuple Fire-Linked Turbo Lasers
250x Concussive Assault Missile Batteries (30 Missile tubes per battery)
250x Single Heavy Ion Cannons
40x Tractor Beam Projectors
500x Single Laser cannons (Multi-role, PD and close range Anti-shipping)
Manoeuvrability:
1,230 g Max acceleration
Max speed 40 MGLT (something around 20 c)
Protection:
2x KDY 72X-SSD Shield Generator Domes
Titanium alloy-reinforced Alusteel with Reactive Durasteel skin-plate.
Targeting:
LeGrange-Pattern targeting computers.
Retribution-Class Battleship

Lore:
The Retribution is one of the proudest ships in the Imperial navy. She has served (like all Imperial Battleships) since the Imperium's birth at the Great Crusade, championing many battles and felling many foes in the Emperor's name. But while the Oberon is the workhorse, the Apocalypse and Victory are the Archers, Invincible the hunter and Emperor the leader: Retribution is the true Battleship among them. She is the toughest and most heavily gun-armed vessel in regular service with the Imperium, as well as being among the largest (like her sister battleships) at around 8 km in length.
The Retributions single design purpose is nothing less than the complete and utter destruction of any brave (or foolish) enough to stand in her path in opposition of the Imperium. Her broadsides being the stuff of legend: pummelling whole fleets into brutal submission in broadside duels, even other Battleships having much to fear from a Retribution's Broadside fire-power.
Armaments:
24x Heavy Macro Cannons (3 Batteries per broadside)
5x Dorsal Lances
6x Heavy Torpedo Tubes**
6x Super-Heavy Mass Drivers (3 per wing)
Unknown number of Weapon Batteries (Melta pulsars, Missile Batteries, Plasma Cannons, Sub-Macro Cannons, Grav Cannons)
*Equipable Torpedo armaments: Plasma Torps (most common), Melta Torps, Vortex Torps (rare), Homing/seeker torps (rare)
**Heavy (larger yield) Warheads
Manoeuvrability:
1,800 g Max Acceleration
Protection:
Multi-layered Battleship-Class Void Shields
Adamantine-Faremite Composite Armour+Alloy-reinforced Adamantine Armoured Prow
Targeting:
Mars-Pattern Targeting Cogitators
Combat Comparisons
Armaments: In terms of Armaments the Executor comes far ahead in terms of sheer volume of weaponry, but the Retribution's lances, Macro Cannons and Torps are far superior to any of the weapons the Executor is equipped with in terms of raw damage per hit. Despite this, the Executor puts out damage ranging into the single Exaton figures, about double what the Retribution is capable of with all of her weapons combined. Although Retribution can more reliably put all of that damage onto a target from a greater range thanks to superior targeting...
So this is a close draw, only because of abysmal Star Wars accuracy: if the Executor could actually hit with a majority of it's shots, then the Executor would be the clear winner.
Range: While the effective range of the Executor is around the same as an ISD (5,000 km), and the Retribution can reliably engage far beyond that (100,000+ km), the Executor (and the smaller ISDs) can hit targets beyond their effective range: their targeting systems just aren't well optimised for it. Normally this would mean a clear victory for the 40K ship, as they can accurately engage from a greater distance where most of the return fire is likely to miss: but the sheer volume of fire the Executor can put down more or less robs that advantage from the Retribution, as the ship is still going to be subjected to double digit Petatones of damage from the Executor (very similar to the kind of damage the Retribution is able to put down on the Executor).
For this reason, neither hold a range advantage over the other, although the Executor would easily overpower and destroy a Retribution with superior targeting equipment or if the Retribution gets too close.
Speed: The two are quite similar here, neither being particularly fast vessels, but the Retribution does hold the edge over the Executor.
Durability:
Now this is tricky to call between the two.
The Executor has the superior shields, being able to withstand impacts from Several Hyperspace-speed Imperial-Class ISD slamming into her and having her shield drop by only 40%, while the Retributions Void shields can 'Merely' claim that it can displace the Earth's mass before finally going down... given the Executor has very few projectile weapons, that's not as impressive as it sounds in this case.
As for hulls, its less of a competition. While the armour of the Executor is exceptional by anything in SW, it's highly unlikely judging by it's inability to resit star fighter impacts for long that it would fair well against Macro Cannons and Lances. On the other hand, the Retribution has been known to brawl with other battleships for hours even without her shield up, armour withstanding all but the very heaviest of fire.
Overall, this goes to the Retribution, as she holds a massive advantage in hull protection and isn't hugely inferior to the Executor for shield strength.
Comparison outside of combat
Both ships are designed as Battleship (one of them quite literally being the purest incarnation of a Battleship in 40K), so neither have many roles to play outside of combat. The Executor can make for a good transport and carries all the equipment and personnel necessary to launch a planetary invasion.
Since the two share identical roles outside of that, the edge goes to the Executor for versatility outside of combat.
The two are extremely expensive and time consuming to produce, but since the Executor means whole populations and planets need to be literally turned inside-out to produce them, I'm giving the small edge to the Retribution here in terms of price and construction.
In terms of speed: Retribution has slight edge in Stellar travel, but massive disadvantage in inter-system travel. Executor wins.
Service life is no competition. A Retribution is considered young if it is a couple of hundred years old while an SSD is lucky if it stays in service for more than a few decades. A Retribution will not need replacing for a VERY long time if not lost in combat, much unlike an SSD, which will wither out fairly quickly in continuous service.
Scenario Battles
Battles between these two ships would most likly be fought as the most epic broadside gun duels you will ever see, with a sizeable fleet escorting each. The SW formation of ISDs, maybe a few Battle cruisers and lesser SSDs/Dreadnoughts along with the Executor will pour out a much more vast quantity of shots: but the IN formation of much fewer cruisers, Light cruisers and BCs with their escorts will be far more accurate and damaging with their shots.
The comparison between the two ships in this combat scenario is no different. At the engagement range where the Retribution is most comfortable, the two will be the equal of the other for fire power, but the Retributions superior Durability will win her the fight. At closer range, where the Executor is most effective, the Retribution will be torn apart in relatively short order by the Fire power they are being subjected to.
Otherwise, whoever's escort fleet wins out over the other will win the battle.
Summary
This one is the hardest one to call, the two are very evenly matched in many cases (despite the many differences in doctrine, age and size). The Retribution holds the advantage at range, but then the Executor rips through the Retribution with ease at close range, for that reason: this is the first draw I'm going to make. The two match each other almost pace-for-pace inside and outside of combat.