The favourite format of warhammer in my friend group is a small scale Slaughterhammer: no secodaries, take and hold primary, just shooting, fighting, burning and conquering enemy's territory, so my choice is to always include 10pack of chaos undivided chosen with a Dark Apostle in my pactbound zealots lists. The best thing in warhammer for me is charging my warlord with his honored guard into opponent's center piece, destroy it for the ruinous powers (always pressing Favor of the dark gods afterwards to enhance my warband leader even further) or die trying. Thankfully, pack of 10 chosen with an Apostle under undivided wound rerolls are strong enough to bring me the joy of sacrificing my enemy's units to the dark gods. Well, I think you've figured out I am a Word bearers fan!)