An Imperial Knight Paladin excels in engagements, where its Chainsword and Shock Lance will make short work of most targets. It is also armed with a Battlecannon for long range support.
The Leviathan is an office block-sized vehicle with an impressive array of weaponry, Void Shield protection, and huge transport capacity. Its main armament, the Doomsday Cannon, has enormous range and will destroy most enemies in a single volley.
Cannons of Khorne (aka Hellfire Cannons) are built with the sole purpose of destroying heavily armoured targets at range. They should be deployed at a location with a good field of view before the battle begins in order to maximise their potential.