Yoaf all,
Lemme try and clear up some confusion.
The Sivar mounted a "Proton Accelerator Gun" (informally "Graviton Weapon"). It fired a bolt visually similar to that of a particle cannon at a target, causing a small explosion. The target's protons are then accelerated, causing a 137 fold increase in gravity... crushing the target (leading to secondary explosions and such). The PAG functions on a planetary scale -- destroying entire cities, installations, colonies, etc. Also of note is the fact that, by its nature, the Proton Accelerator Gun completely ignores artificial (phase and meson) shields.
The Kilrathi used the weapon to destroy their installations in the Warhammer System... and then mounted it onboard the Sivar, which destroyed the human colony at Goddard (and we all know how that story ended).
After the Sivar was destroyed (on 2655.199), Confed salvaged the weapon and quickly developed their own version: the Phase Transit Cannon. The PTC is a tradeoff compared to its Kilrathi ancestor: it does far less damage, but can be used against moving capital ships.
Wanting to put the weapon into service as soon as possible, Confed quickly redesigned their Concordia SuperCruiser replacement, basing it around the PTC. That same year the first hulls for the Confederation-class dreadnought, with the PTC as its keel, were layed down. They began entering line service in 2660 (with the Concordia herself coming on line in 2661).
Does this make the Concordia the 'most powerful ship'? Certainly not -- the PTC didn't really work very well. It was discovered that it had a tendency to heat up and explode rather than fire at an enemy ship... so production of the Confederation class ceased in 2665, and use of the weapon (for the most part -- you all know the exception) was halted.
Why aren't there any more Sivars? A few notes...
* There technically are -- the same hull is used as a standard dreadnought (sans-PAG), and a smaller pocket-carrier version is also known to exist. There's also another, earlier dreadnought called Sivar by the Kilrathi.
* In 2655 Task Force Delta, lead by the Tiger's Claw, attacked and destroyed the facilities where the Sivar was fitted with the PAG, seriously setting back production.
* All that said and done, Confed believes the Kilrathi *did* continue to produce PAG-mounted Sivar class ships (Joan's 2664 Update)... but that instances of their use against humans are simply unrecorded or non-existence.