My favourite weapon in the Mechanicus arsenal is the HUMAN MIND.
j/k, I have a real answer, and it is the Irradiation Engine in 30k. Here's why.
30k is mainly marine vs. marine action. People generally think of Marines with different marks of power armour, especially Legion-specific specialists, often in huge infantry groups as if reproducing the covers of Horus Heresy novels. The Irradiation Engine targets specifically that, melting past normal 3+ armour, wounding on a 2+ and it's a torrent weapon so the template can be maneuvered all sorts of silly ways. There is a terrible beauty bore by the faces of opponents who bought those Legion-unique Marine units or the FW shoulderpads or what have you and have them literally wiped out with a swipe of your hand. Not good against Terminators or Primarchs or Vehicles, though.
I am sorry to drink their tears, but if I was not meant to, then why did the Omnissiah make them so delicious?
It's not about being powerful to the point of OP, it's about how the Omnissiah teaches us, it's "the right tool for the job". The Irradiation Engine in the context of 30k is about as specific an example as you can get. The Machine God's lessons are so beautiful.