If you use the forgeworld rules for a chaos knight, you can give it things like mark of nurgle (it will not die), mark of khorne (re-roll stomps), a dirge caster (no overwatch within 6"), and it comes stock with a foe-reaper chainsword, which gives it +1 on the D-table against monstrous and gargantuan creatures. Its what I use, normally the paladin though for the 72" rapid fire battle cannon.
"That which we foolishly call truth is only a small island in a vast sea of the unknown. For man to truly flourish, he must be willing to abandon the ever-shrinking island of such petty 'truth' and surrender himself to the reality of that which is beyond."
Dark Apostle Erebus, architect of the Horus Heresy