Melta gun...all the way. But modified for Slaanesh to make it a bass cannon. Destroy your enemies with organ rupturing bass!
I will carry my power scythe 'Verificare ex Mortem' made out of the spine of an ultrasmurf force commander and my heretical texts and runes that empower my power armor because... My dark and unholy ones my true weapon will always and forever be THE WARP!!!!!!!!!!!!!!!
I wonder if there are any big-ass two handed Power Swords or something in the lore. That would be the perfect weapon for me to crush the Astartes!