system
Given the following post, respond as if you're commenting on Reddit:
human
[Culture Series] just how powerful are The Culture?
gpt
The Culture are on of the few Level 8 civilizations currently "in Play".  Generally there are 4-9 Level 8 Optimae around at any given time.  In general any level 8 will trounce a level 7 civilization, and so on down the line.

Their are more powerful forces than the Level 8 civs.  The Sublimed for example are civilizations that have moved on from the physical realm and transcended to another plane.  They are thought to be incredibly powerful, but also uncaring of what happens in the Real.  

Elder Civilisations are ones that have moved on from normal galactic politics and want to be left alone, wherever that may be (home planet, other places, artifacts, whatever).  Best not to trifle with them.

As to how powerful the Culture is, they are normal or above average for a level 8.  Casual planet busting is a good description.  Any old GCU (General Contact Unit, an exploration and scientific ship) can waste a solar system.  Actual OU (offensive Units, or later called Picket Ships) are much more deadly.  GSV (General Systems Vehicles) could take on millions of ships from less advanced civilizations, a war prepped GSV would be a match to fleets of equivitech opponents.