Devastator. If we're talking Gause firepower.
That was an SLDF mech IIRC.
The
Devastator was incredibly rare and only seen in the very last stage of the invasion of the Amaris Empire. The
Highlander was widespread, being used by every House, available to mercenaries, and (therefore) to the Periphery states who hired these soldiers.
I would say the
Pillager is a much more analogous 'Mech in the Star League era, having been around since the Reunification War. Only the Periphery states and Draconis Combine made no usage of it.