...the Death Star, an armored space station with
enough power to destroy an entire planet
-Star Wars: A New HopeThe overwrought super-weapon defines Star Wars so completely that it appears, in various forms, in three out of the seven films (and is referenced at least once in the prequels). You cannot have Star Wars, or a knock-off, without having some. They certainly fall into our model of "Star Wars as World War 2" with the Death Star taking the place of the Nuclear Weapon.
SS3, on page 23. has the Azrael-Class World-Killer, but it's too hard-science for our tastes. If we allowed one ship to reach near-C velocities and then shatter a planet when it hit it, then nearly any ship could do the same. The purpose of a Sword-Class heavy cruiser would be as a galactic WMD, which isn't our intent.
The same rules suggest the (TL 12^) Adversary scaled up to SM +33, with cosmic power. Psi-Wars isn't TL 12, nor does it have cosmic power options, but let us consider, as a thought experiment, what such a weapon would look like.
An SM+33 vessel is so large, it is effectively 6 size categories larger than a dreadnought. It would have a -18 to hit an Empire-Class vessel with its main cannons, and its tertiary batteries would be at -14 to hit a fighter. And that's okay, because a tertiary battery with a Very Rapid Fire UV laser option would still be firing a 3 petajoule weapon, dealing 3dx1000(5) damage: A single shot from the most minor cannon possible (barring the "peripheral battery" optional rule) would instantly vaporize a mythic-model Empire-class dreadnought. Which is fair enough, because one such cannon (never mind the whole battery!) would cost half a quintillion GURPS dollars, and weigh 5 billion tons. To put that in perspective, one cannon weighs more than five thousand empire-class dreadnoughts.
The scale of such a ship staggers the mind. The real is the enemy of the cinematic in a genre like Space Opera, and the energies required to destroy a planet are realistically mind-boggling. If you apply such a premise to a ruleset like GURPS Spaceships, terrifying realities begin to spill out. Consider, for example, the computer in such a ship. The control room would have a computer with complexity 22, which is sufficient to run up to two complexity 22 programs, one of which is sufficient to run a volitional AI with an IQ of 38. Even if we argue that the control room can afford to be smaller (how much handling do you really want on a ship like this anyway?), you're still running around with an IQ of 35+. This "ship" begins to look less like the Death Star and more like Unicron, devourer of worlds.
If too much reality begins to intrude into our setting, we can either embrace it or we can just ignore it. Starkiller Base looks more like a small planet than a large space station, which is appropriate: Such a starship would be a world unto itself. It would be powered by a powerful, alien intellect, and it would be utterly ineffective at destroying anything smaller than a planet (requiring agents, such as an entire civilization's worth of "smaller" craft, to deal with anything smaller than planets). The weapon itself might not even instantly blow up planets, but inflict horrendous damage on a world turn by turn, literally carving it to pieces while its oceans boil and terrible storms flay the inhabitants from the planet. The Adversary would travel the galaxy, wreaking terror and havoc until such time as it was finally destroyed by some unknown means
Alternatively, we ignore all of this. The spaceship design system was never meant to handle such a vast ship. You don't have to build giant cannons or continent-sized computer brains in such a station anymore than you stat up a planet with weapons scaled to 1/20th the size of the planet. Such a space station is really just a giant, mobile location that has the capacity to blow up planets, and has some capacity to deal with fleets. Treat it as an effectively immobile Empire-Class dreadnought with the equivalent fire power to, say, a hundred Empire-Class dreadnoughts, but you can only bring a single Empire-class dreadnought's-worth of firepower to bear against a given ship or squadron at a time. You need some secret macguffin to deal with it, though what is needed, precisely, depends on the nature of the super-weapon. Most of the time, the simplest way to destroy such a vessel is simply to allow its own energies to unravel and destroy the whole ship "in a chain-reaction" or some such.
This may seem unsatisfying, but that's exactly how Star Wars itself deals with super-weapons like this and, practically, there is no other way to deal with it. You can't really fight something that throws out zetta-joules of energy, not with something that only deals with mega-joules. It's like trying to fight wrestle a black hole with Jupiter.
Other Super Weapons
GURPS abounds with neat setting ideas and catalogs full of interesting items that everyone promptly forgets because they don't have a context to work them into their particular setting of choice (the ideal example of such a setting is Dungeon Fantasy, which is one reason why it's such a strange line, because it 180s away from the typical "rules, not catalog" rule-of-thumb that dominates other GURPS lines). We have that context, though, so let's take a look at some potentially interesting super-weapons.
Found in Pyramid #3-37 on page 4, J. Edward Tremlett has an article regarding a small planet that's been turned into a stellar-former par excellence, the Starmaker. The setting in the article is more strongly focused on science and genetic engineering than is, perhaps, appropriate to Psi Wars. It better suits something inspired by Mass Effect or Star Trek, but it could work in Psi Wars. The Starmaker is the sort of megaproject one might imagine from a galactic civilization, and fleets of warships, dangerous assassins and the uncovered secrets of ancient xeno-civilizations certainly fit a space opera universe, so why not ours?
The Starmaker could serve as an interesting alternative for the Death Star: Once an innocent civilian weapon meant to improve the lot of man, it has since been turned into an engine of destruction by a sinister empire that seeks to use its might to cow those who oppose its might. We get the light within the dark motif so common in Star Wars. The fact that it's (almost) an utterly unique artifact only heightens its appropriateness for the genre.
Unfortunately, it lacks stats necessary for actually shooting it down, but given its size, treat it as the Adversary above, though probably with less firepower.
The System Shell
Pyramid #3-9 has an article, starting on page 4, that's chock full of ideas for a space opera campaign. The System Shell prevents people from escaping from a star system. Technology similar to the System Shell already exists in the Star Wars universe in the form of the Interdictor-class Star Destroyer. We could apply a similar approach in Psi Wars: Treat the System Shell as a technology that requires an SM+15 (or three SM+14) systems. It costs 10x as much as a SM+15 Stardrive. The System Shell, while powered, either prevents escape outright, or forces a Mechanics (Starship) roll to "charge the drive" at -10. Ships in hyperspace that cross the System Shell are immediately shunted back into normal space, meaning that one can "jump into" the system, but not "back out." When the System Shell is done, the ship can switch it off, and its fleet can simply jump out.
The System Shell becomes a prime tool to create choke points by forcing people to stop and fight in a particular system. If you're aware of where a System Shell is, you could bypass it by navigating to systems around it, but the System Shell is ideally placed on a hyperlane, where hyperspace travel is easiest, at a choke-point where travel to other systems is exceedingly difficult.
The Madness Bomb
Steven Marsh's excellent Doomsday Weapons articles continues with another gem perfect for Psi-Wars: Given that Psi-Wars is about Psi, a weapon that either creates (or destroys) Psi and leaves madness in its wake might fit Psi Wars perfect. Of course, this requires some assumptions about Psi that we don't have yet (and a bomb that infused everyone with the Force in Star Wars would be... weird and inappropriate), but we should definitely consider it as a dangerous option.
A super-weapon doesn't have to destroy worlds or wipe out civilizations to be a super-weapon. A new or innovative ship that does something previously unseen might be more than enough power.
The Adversary-Class Super-Dreadnought, Redux
So we already discussed using a scaled-up Adversary-Class Super-Dreadnought as a World-Killer. But what if we just used the Adversary-Class Super-Dreadnought right out of the box. How would a TL-12 Super-Dreadnought impact the Psi-Wars universe? Like an incarnate God, that's how. It's armor and shields would make it impervious to any other existing ship, and it's 500G subwarp engine, even if we quarter it because it's reactionless and on a dreadnought, would still give us around 175gs, or +7 acceleration bonus, allowing it to keep up with quite a few fighters!
In short, it's exceedingly over the top. A more modest version, which we might call the Star of Morning model, might be essentially the Empire-Class Dreadnought: Mythic Model with a single change: Give it a cosmic-powered TL-12 gamma-ray laser spinal weapon, and make one additional change: Give it a 1/3 RoF. When the Star of Morning shows up, it needs to spend 2 turns obviously and dramatically charging its weapon before it can fire. Once it fires, it does so with a staggering 3dx1000 (10) damage, That'll penetrate an Empire-Class dreadnought's defenses with ~9000 damage to spare, utterly evaporating it in a single blast. It would represent a terrifying weapon that would utterly dominate fleet battles, and represent a unique, experimental and sabotage-worthy device.
The Vixen-Class Patrol FrigateHere's an even more modest idea: A typical TL 11^ light corvette that already fits perfectly, without much modification, into the Psi Wars universe with one exception: It carries a Cloaking Device, a technology not available to any other vessel in the setting. Assuming that the Vixen is the only vessel to carry such technology, then it becomes a ground-breaking weapon that will rapidly change warfare throughout the Psi Wars universe. Being a light corvette, it's also the perfect size for a small crew of 4 elite individuals to fly on, say, secret missions.
It should be noted that the Vixen-Class Patrol Frigate, stripped of its Cloaking Device (Replace that slot with armor, to give it 30/30/30) makes a decent light combat corvette.