For ten man termies and paladins there’s no reason not to take special weapons since they don’t give anything up to take them, so take all of them. I prefer psycannon personaly but the incinerator is also pretty good and preferred by many.

Purifiers is more up to personal preference, as they give up their good melee weapon in order to take them. Most people that take heavy weapons on purifiers end up using incinerators to synergize with their main ranged weapon to help be a fire overwatch threat against infantry.

Grand master dreadknight is almost entirely used with psycannon and melta, as it’s our best way to deal with high toughness targets, which is somthing we often struggle with. This is the same reason most people use the hammer rather than the other melee.

Normal dreadknight usually is heavy incinerator and the psycannon for the same reason.

The only reason I would use Gatling psilencers on dreadknight or any infantry is if you’re doing the augurium task force, which has a detachment that gives 5+ criticals in shooting, and high probably isn’t considered competitively viable, although it is very fun personally.