Which is the most robust department?

Science: Research gamer tech
Medical: Apply cyanide in syringe gun, also plasma smoke grenades
Security: Roundstart access to guns and armor
Cargo: CARGONIAAAA!!!
Service: Death nettles, cqc, shotgun,
Engineering: fusion flamethrower, trash cannon
Assistants: Pipegun and elite pipegun

Service because watching chaos unfold as a janitor just never seems to get old

In terms of destruction I’d say Atmospherics subdepartment, because you can unleash hell over the whole station by turning on a single pump.

cyanide

Oh you sweet summer child… you have no idea what our evil chemists are capable of.

Try a syringe that puts you to sleep and kills you in 8 seconds
or one that autohusks you in like 6

Cargo because no matter the tech they still control your resources and the QM can just cut out your ore silo access