Sicaran Battle Tank
The Sicaran Battle Tank was an awesome heavy tank that was used by the Space Marine Legions near the end of the Great Crusade and during the Horus Heresy. Created by the combined brainstorming of Ferrus Manus, Roboute Guilliman and some Adeptus Mechanicus Magos members (with the help of some elements derived from Rhino and Land Raider STC's), the Sicaran was designed to be a fast moving tank hunter that would complete both the Predator Tank and Land Raider. In fact, it was so good that Horus himself made sure that a large number of these tanks were at his and the Traitor Legion's disposal before he got on with his rebellion.
Armed with two Herakles Accelerator Cannons, two sponson-mounted Lascannons (as well a forwards mounted Heavy Bolter), nicely armored, swift for its size, and equipped with systems that allows it to hit fast moving objects and right where it would hurt the enemy most, the Sicaran Battle Tank was a valuable asset for the Marine Legions.
Given that the Sicaran was made from the best bits of at least two completely different STC vehicles, the Imperium would still do that to gain a huge advantage over their enemies.
Variants
Other variants (albeit only one...for now) include the Venator (that's Latin for "Hunter"). Swapping its Accelerator Cannons and Heavy Bolter for a Neutron Laser Projector, this Sicaran variant was even more efficient in its role as tank destroyer. The lack of a turret (which is standard for the vanilla Sicaran Battle Tank) meant that the vehicle had to turn its whole self to face a enemy (which could be fatal when it comes to fighting enemies from multiple directions, especially when taking into account that the reactor for the neutron laser was highly unstable and tended to explode violently if the vehicle was heavily damaged).