The infamous Borg Cube is the most powerful and largest ship known to exist. In assimilation attempts the Borg typically send just one Cube since that is all that is usually needed to accomplish their goal.
The first encounter between a Starfleet vessel and a Borg Cube was in 2365 when Q sent the Enterprise-D to System J-25. From that encounter Starfleet learned what a Cube was capable of, regeneration, powerful weapons, and incredible warp speeds. In the next encounter Starfleet sent 40 ships to intercept a Cube heading for Earth, 39 ships and 11,000 lives were lost at Wolf 359. Their second attempt at Earth in 2373 also failed but not before several Starfleet vessels were destroyed. Aside from these major battles, other small incursions have been observed; several Federation and Romulans outpost were completely destroyed in 2364 by a Borg ship.
Despite the power of Cubes, they were easily destroyed by Species 8-4-7-2 bioships in 2374 which were powerful enough to destroy Borg planets as well.
Like everything else regarding the Borg, much is unknown about Cubes, they have a mass of 2.5 million metric tons, and one face of the Cube is five kilometers square. The inside of a Cube is a twisting labyrinth of corridors and catwalks. The interior features assimilation chambers, thousands of regenerative alcoves where the Borg control the ship and maturation chambers were infants and childeren of species stay until they have matured enough to serve the Collective. There is a large docking bay able to house ships. Cubes also carry smaller scout Cubes and Spheres.
Model concept by Maurice Hurley, design by Rick Sternbach, built by Kim Bailey.
Interior of a Borg Cube