The Ark Mechanicus is a capital ship fielded by the Adeptus Mechanicus. Part Battleship, part city-sized laboratory and factory complex, and part spaceborne shrine to the Omnissiah's might rolled into one, these venerable ships are often seen as part of Explorator Fleets, plying the spaceways in pursuit of lost knowledge.
No Ark Mechanicus is alike, as each is an expression of the preferences of the Archmagos that commands it. What is sure however is that each features systems far advanced than any comparable ship in the Imperial Navy.
Notable Ark Mechanicus
The Ark Mechanicus of Archmagos Lexell Kotov, the Speranza was actually found half-completed underneath one of the forge worlds that the magos oversaw. Kotov restored it to working order, and with it led an expedition to the Halo Scar to investigate the rumors of a Dark Age of Technology artifact called the Breath of the Gods supposedly located there.
The Speranza itself seems to predate the Dark Age of Technology, and features a powerful if somewhat secretive Machine Spirit, plus an unknown number of secret technologies far more advanced than anything the present-day Mechanicus can even dream of creating, such as a reactor that uses exotic anti-matter and graviton pumps to draw power from dark matter. The most infamous of these are a black hole cannon that aim accurately in the middle of an electromagnetic storm, a gun that rewinds time so that its target destroys itself as its past and present versions collide, "chronometric cannons", "anti-matter projectors" and "hypometric weapons" so powerful it can cause vast regions of space to just not exist.
It was also especially large, even for an Ark Mechanicus, being large enough for a maniple of Titan to manuever inside with room to spare, along with millitary exercise yards big enough to simulate a small city.
Fittingly these technologies only make themselves known when the ship itself allows it, and are unusable otherwise. The Machine Spirit is actually a full AI (or something greater) with a complete STC database, and just chooses to lay low and wipe the memory of those who interface with it.
The Ark Mechanicus of Archmagos Belisarius Cawl, within the depths of this massive freighter-hulk slept the countless Primaris Space Marines that were created since the magos was given his secret mission by Roboute Guilliman ten millennia before. Upon the Primarch's return, this vast host was roused for the Indomitus Crusade.
After the last of the Primaris had been distributed to the Astartes Chapters that needed reinforcement, the Ark became Cawl's mobile headquarters, with the attention of the magos now turned to finding an intact Pylon network somewhere in the galaxy to research, in hopes of replicating the technology and use it to close the Great Rift.
As the Belisarius Cawl: The Great Work novel reveals, the Zar-Quaesitor is modular, and its entire front section can detach into a planetary lander that's larger than an Astartes Strike Cruiser. Said lander becomes Cawl's base of operations for his little foray onto Sotha.
In Battlefleet Gothic
An Ark Mechanicus is normally equipped with a Nova Cannon, four Lance batteries, and two Weapons Batteries. This makes it an absolute terror at long ranges, but somewhat more vulnerable at close range due to its lack of point defenses or fighters. Said vulnerability is actually quite misleading because it's lacking only compared to other AdMech battleships with improved turrets. The point-defense of Arc Mechanicus is the same as any non-carrier battleship out there, and non-elfdar bombers will have a hard time getting through that 4-turret-stack.
Despite this each Ark is easily as tough as a Battleship, with armor and shields to match.
In Battlefleet Gothic: Armada 2
The sole Battleship of the Mechanicus fleet, the Ark Mechanicus is geared more toward long-ranged combat thanks to its abundance of Lance and Plasma weaponry. Its also quick too, able to keep pace with Cruisers, though don't expect the thing to turn on a dime.
On the flipside, the Ark Mechanicus features the same big Leadership penalty that all Mechanicus vessels have, which means its especially vulnerable to boarding actions, and no, we don't get the Speranza, hopefully in a DLC campaign?
Close combat problems can be somehow attenuated by using stasis bombs and the Eye of the Omnissiah Nova Cannon shot to delay any enemy charge, if you are playing the campaign on annihilation mode just sit near the border of the map with lock-on, spam the Nova Cannon and once things become dire order a retreat, this is particularly good against factions such as orks and tyranids since they are going to rush-ram your ships, even if you evacuate the damage unleashed shall be enough for the rest of your fleet to mop things up.