It'd either be a slightly altered version (need FTL drive!) of the PSNS Truth from my Gundam Wing fanfiction, or a fully tricked-out Enyo from EVE.
Honorable mention must be made to both the GTD Soyakaze and the GTVD Unity as my FS homes away from home.
I've often tried to model Truth, but never succeeded to my satisfaction.
Truth is massive. I've never assigned it a specfic size in the story, beyond "many kilometers". Mentally I've always envisioned it as between ten and twenty. For appearance, think somebody cut off the superstructure on a Star Destroyer and left just the wedge-shaped part. Also eliminate the docking bays.
Truth's main battery consists of 36 90cm railgun triple turrets, capable of propelling projectile weighing close to fifty tons at a significant fraction of the speed of light. Secondary batteries consist of 60 twin 15.2cm railgun mounts and 90 singly turreted 10.5cm railguns. For anti-aerospace and point defense, Truth has no less then 240 quad-mounted 40mm Mark III beam cannon, each tied into it's own Mark 34 Fire Control System, and backed up by an array of 60 VLS light missile launchers.
As if all this were not enough, Truth can also deploy no less then four complete space mobile suit wings from its MS bays. This breaks down to 24 squadrons of 18 SMS each, for a grand total of 432 suits, either upgraded Tauri or Serpents. It can deploy half this force within 45 seconds thanks to their being stored in special quick-launch tubes. (Think BSG, but much shorter.)
Truth's armor plate is Gundanium, five meters thick on the first exterior layer, and Truth is triple-hulled. As might be expected, the ship is not capable of impressive acceleration.
Despite all this, the Truth is not invincible. In the end, in the story, it succumbs to massed firepower from the Savo, Surigao, Dogger Bank, Calabria, and Empress Augusta, battlecruisers all. But until then, it was the most dangerous thing going, practically indestrucible by any conventional or even non-conventional means.