Imperial Navy Battleship
"A magnificent ship no doubt, but I would rather have a crew who knew their airlaps from their transons."
- – Admiral Rath, on viewing the Emperor-class Battleship Dominus Astra
The biggest and heaviest ship class available to the Imperial Navy, the Battleship is often the flagship of the Admiral during a campaign. These ships are often the heaviest hitters of any armada,with guns, armor, fighter and bomber support, and torpedoes a-plenty. Properly supported by its fleet, a Battleship can easily turn the tide in an engagement. Just don't expect them to get anywhere quickly, due to their relatively ponderous speed.
Many Battleships are centuries or even millennia old, with rolls of honor to match. The loss of any Battleship is keenly felt by the fleet in question, with each a near-priceless relic that cannot be easily replaced, even by a Forge World like Mars.
Battleships typically come in three flavors: Ordinance ships (armed with a majority of Macrocannons), Lance ships (armed mostly with Lances), and Carriers (carrying mostly Fighters and Bombers).
Classes
Carriers
Emperor
The poster-ship for Battlefleet Gothic, the Emperor carries four Launch Bays worth fighters and bombers, which are supplemented by a suite of macrocannons for ship-to-ship combat. This means that the Emperor can reliably put the hurt on enemies at extreme range, and in-universe accounts for the versatility and self-sufficiency of the class.
In Battlefleet Gothic: Armada 1 & 2, the Emperor has the longest Identification Range of the ship type, shared only by the Oberon-class Battleship and Widowmaker-class escort.
Nemesis
A modification of the Emperor-class, the Nemesis adds two more Flight Decks, to a total of six, in exchange for some of its guns. Another rare class in-universe, as all existing Nemesiswere created from damaged Emperor-class ships.
Lance Ships
Apocalypse
The long-ranged sniper of the Battleships, the Apocalypse carries Nova Cannon and Lances a-plenty, this makes it the bane of small fast ships. In-universe it's one of the oldest Battleship designs available, and fairly rare, due to the Imperium being unable to replicate the Lance tech it uses.
Ironically in Battlefleet Gothic: Armada 1 & 2, the Lance batteries of the Apocalypse had a shorter range than its Battleship brethren. One had to use its Overload Lances ability to bump up the range of its batteries up, which makes no sense to people, as it should have had those ranges at base.
Victory
An attempt to recreate the Apocalypse-class with less fancier tech, the Victory is essentially a Retribution with its Broadside macros replaced with Lances, and its Torpedo tubes replaced with a Nova Cannon. Much like the Apocalypse, it's good at sniping, and getting rid of pesky escorts.
Ordinance Ships
Invincible
A "fast battleship" class, the Invincible was designed with the pursuit of Xenos and Chaos raiders in mind. Featuring the speed of a Dauntless on top of the usual weaponry of a Retribution, the Invincible was quick as it was deadly.
Unfortunately this speed came at the expense of protection, which meant that in a straight battleship duel the Invincible-class crumpled like wet cardboard (also not helped by Admiral's foolishly using as front line sluggers). Despite this, the ship was deemed successful enough to mass-produce.
Retribution
What one gets when you cram as many macrocannons on a hull as humanly possible, with some lances and torpedoes for flavor, the Retribution is as close to the Imperial Navy can get to a bruiser, capable of comfortably duking it out with opponents at medium-to-long ranges.
In Battlefleet Gothic: Armada 1 & 2, the Retribution is actually the fastest Battleship type available to the Imperial Navy (probably as a nod to the Invincible-class). It also has arguably the highest damage potential of the available Battleships in the games; a full broadside plus successful ram is capable of crippling or even destroying ships the size of Cruisers or smaller.
Others
Oberon
The Hybrid of the Battleships, the Oberon features heavy Lances, Macrocannons, and two fighter bays, on top of its usual Torpedos. A somewhat rare class of ship in-universe, the Imperial Navy still considers the Oberon a success due to its self-sufficiency. While it may not be able to duke it out in a straight fight with other Battleships, its armaments make it a terror to smaller ship classes.
In Battlefleet Gothic: Armada 1 & 2, the Oberon shares the same Sensors ability that the Emperor has, increasing its Identification Range. This means that it can work in tandem with its Carrier cousin and support its fighter and bombing runs, while at the same time being able to snipe with its Lances.
In Battefleet Gothic
Powerful, well-armored, and well-shielded, Battleships are the centerpieces of a fleet, regardless of point size. However they're are also slow and expensive (you can get about two Cruisers for one Battleship).
Positioned properly and kept adequately screened by smaller ships however, and Battleships are well their worth in points.