Leviathans

Leviathans are the biggest constructions ever attempted by Humankind (and the species that follow its general outline of ship classes) - at least when you consider the ones supposed to move and shoot enemies. Constructing a single one of those giants requires an intense amount of money, workforce, resources and time - not to mention technology that is required to make such constructions (almost bordering on the edge of megaconstruction) feasible.
It is rare for a single country to have more than one Leviathan at the same time. The only exceptions are countries that are almost perpetually facing a threat of certain extinction (for example by being attacked by particularly xenocidal aliens), and those extremely strong (Terran Federation, or countries controlling a whole star sectors/significant parts of them.
In most cases they serve as a navy flagships and/or carriers of the most potent weapon systems in existence, ones supposed to be able to single-handedly decimate fleets. They spend most of time near the capital worlds, only moving when they are really required. They are essentially a glorified, movable bunkers that serve as a last line of a country defense... but also ones that can move and spread destructions in a critical systems.

Variants

Colossus - Colossi are smallest of the Leviathans. That, despite this, remain a massive ships capable of causing absolutely insane level of destruction. Due to being a relatively small ships they aren't big enough to be armed and equipped in a versatile, universal way. They tend to be internally divided between carriers of Fleet Killers - ships literally built around a massive weapon system - and the more conventionally armed warships. That in most cases are armed in an array of spinal-mounted MAWs (themselves almost on a level of Fleet Killers) and a variety of conventional weaponry, that essentially makes them an oversized dreadnoughts. Their tonnage is normally between 10 and 15 million tonnes. They are (on average) ten to fifteen kilometres long and have a crew up to 30 000 people.
Titan - Titans are the medium form of the Leviathans. Average in size, average in speed, average in amount of weapons they can carry. Constructing a single one requires massive investments even from a major Outer Colonies states. Their cost is truly insane, and creating a Titan might take up to a decade (depending on how determined its creators were). They are commonly armed in a Fleet Killer class weapon, an array of spinal mounted MAWs and a wide variety of conventional weapon systems - plus, of course, an ability to field a vast number of smaller ships. Their tonnage is normally between 15 and 40 million tonnes. They are (on average) twenty to twenty five kilometres long and have a crew up to 70 000 people.
Goliath - Goliath is a pure symbol of madness. Only a few countries have economies capable of creating and sustaining one of those gargantuan doombringers - it is said that aside from Terran Federation, entire Humanity has at best ten of them, most of them in service for at least a century. Their firepower and resilience knows no bounds. Each of them is capable of destroying a fleet on its own. It's a dreaded warmachine capable of bringing undescribable destruction. They always carry either a single massive or two-three smaller Fleet Killer class weapons, a vast array of spinal mounted MAWs capable of shredding through a dreadnought armour like through paper, a massive amount of conventional weapons and entire fleets of LACs. Their tonnage is normally between 40 and 55 million tonnes. They are (on average) thirty to fourty kilometres long and have a crew up to 130 000 people.