Sanctuary II: Heavily armored and armed interstellar starship.
The Sanctuary
REVISED! - This mighty starship (and its duplicates) which measures nearly a mile across, was once the the Home of Thanos after his exile from Titan. Built sometime after he had been exiled from his Titan, this space-ark essentially serves as Thanos' moblie Headquarters. Its carries highly sophisticated sensors and scientific instruments, science and weapons labs, living quarters, a large hangar-deck filled with fighter craft, hyperdrive engines for rapid interstellar and intergalactic transit, and it of course possesses enough armaments and power to lay waste to an entire planet.
In the battle between Thanos and the original Magus, so much of this original vessel was destroyed (almost half of its total mass), that virtually the entire ship had to be rebuilt and refitted. After it's reconditioning, the ship was dubbed the Sanctuary II. Later, during his gambit of stellar genocide it was revealed that Thanos had built an exact duplicate of this vessel, dubbed Sanctuary III, and used the duplicate as the flagship for his armada in order in order to lead the Earth forces astray. The true Sanctuary II was hidden on the far-side of Earth's Sun, where Thanos attempted to use his Stellar Projector to destroy the star without interference. However, this plan was foiled by Captain Marvel, Thor, Iron-Man, and also Warlock who died in the attack. Eventually, Thanos was slain by Warlock's vengeful spirit, and the Avengers and Mar-Vell were successful. However, much of the mercenary Armada escaped, and the Sanctuary III evidently vanished as well.
For unknown reasons, the Sanctuary II was abandoned, and left to drift in Space for some time. However, it would later be taken under the command of Nebula, who used the claim of being Thanos' granddaughter to take charge of the ship. She didn't command the ship for very long before she ran afoul of the Avengers and a contingent of Skrulls, and she was eventually deposed and the ship was emptied of its crew. Starfox then disabled the ship's gravitational stabilizers, and the Sanctuary II apparently destryed itself. However, things were not as they had seemed. Unknown to everyone, the disembodied AI program called Quasimodo had chanced upon the Sanctuary II before anyone else had arrived. The process of assimilating its control-systems was slow, but by the time Nebula had been defeated, Quasimodo had taken control of enough systems so that he could holographically simulate the ship's destruction. He then piloted the ship back to Earth's solar system so he could complete his integration without any further interference.
At some point Nebula would gain control over the Sanctuary III and assume command over Thanos' former mercenary fleet as well. For a time she remianed a meanace, until something happened that nobody expected: the resurrection of Thanos at the hands of Death herself. Following this rebirth, Thanos had been given a mandate to kill half of all life in Death's name. As part of his plan to do so, Thanos dispatched Nebula and re-assumed command over the ship and his former fleet. Though it's fate after the Infinity Gauntlet affair is unknown, however it is likely that Thanos retained control of the vessel and had it secreted away.
Some years later, Quasimodo would return, and use the Sanctuary II in a gambit of revenge against Spider Man and the Silver Surfer. His plan to download his mind into the body of the Silver-Surfer was foiled, however, when Thanos appeared and took command of the ship's systems, and quickly expelled Quasimodo. Afterwards, Thanos allowed the Surfer and Spider-Man to leave, and took his ship to parts unknown.
Since then, several versions of the Sanctuary have been captured or destroyed in several of Thanos' subsequent adventures. Here's a tally (the number-designations are speculative):
Sanctuary ? - A seemingly modified version. Used by a Thanos duplicate duing "Celestial Quest". Taken by the Avengers.
Sanctuary II - Used during the Walker story-arc in "Captain Marvel". Later destroyed by a singularity-trap at the hands of the Thanosi during "Infinity Abyss".
Sanctuary III - Used by Thanos and Gamora for the remainder of "Infinity Abyss". Later used during the events of "The End", and the "Epiphany" story-arc. Destroyed by Galactus.
Sanctuary IV - Summoned by Thanos for the climax of "Epiphany", and used in the "Samaritan" story-arc. Destroyed by The Fallen.
Because Thanos has shown to be capable of replacing his ships rather quickly once they have been destroyed, he almost certainly has a hidden shipyard somewhere, possibly sequestered in a large asteroid field. Such a location would provide adequate cover, and also a near-limitless supply of raw materials for shipbuilding. Even so, in his last series of adventures he seemed to go through versions of this ship very quickly, probably too quickly for them to be so easily replaced. Since he mentioned to Warlock that he had used the last dregs of the power he had gained from the Heart of the Universe to replace his armada of robotic ships (Destroyed in "Infinity Abyss" #6), it is likely he also created one or more Sanctuary duplicates as well.