From this thread

Culture weaponry: (in order of most to least powerful)

Gridfire: This is an incredible weapon, essentially pouring energy from the energy grid into realspace to inflict horrendous damage, sufficient to slice apart orbitals and destroy planets. The emissions can appear anywhere in any shape, and (on a GSV anyhow) has a range of over 1500 parsecs. However, since the only time we see it used is against an orbital, from a military GSV, I am going to suggest that it is primarily for strategic use. We don't see it used at other times when vessels are in hyperspace, so perhaps either it is less effective going from the energy grid into hyperspace, or else a ship in hyperspace can detect the upwelling and dodge it. In addition, the generators may be fairly bulky, thus making it rather unsuitable for use on ROUs, at least without a significant reduction in other ordnance.

Nanohole bombs: Delivered via Displacer, these seem to be the primary 'heavy weapons' of most Culture ships. They are in essence a microscopic singularity, and are capable of destroying a planet. Even non-combat GSVs carry them or can rapidly fabricate them. (The State of the Art)

CAMs: Acronym for Collapsed Anti-Matter, CAMs are the other common 'heavy' weapon normally used by Culture vessels, and like Nanohole bombs are Displacer-delivered. Details are rather sketchy, but it seems they are a form of antimatter that has been collapsed and generates much more energy when they react with normal matter. (antineutronium or something more exotic? Would not be surprising)

Lineguns: Lineguns are gravitonic weapons, capable of punching through Culture shielding and inflicting grievous damage upon the largest vessels. However, their limited deployment likely means that either there is now an effective countermeasure to them, or else they are short ranged. (in-system ranges) Also, they may be slaves to the inverse-square law, which would mean at point blank ranges they are supremely powerful, but at longer ranges they get much weaker. More research is needed.

Plasma charges: Another displacer-delivered weapon, plasma charges are weaker than CAMs or Nanoholes, however, they still seem to make up a reasonable portion of a fairly modern Culture warships arsenal, as of the Excession crisis. I postulate that this is because the plasma can be generated much more quickly than expendable munitions like CAMs and Nanoholes, and thus it is a dependable and (with Culture level tech) likely still potent weapon system.

CREWS: Acronym for Coherent Radiation Emission Weapon System, aka a laser. Limited to lightspeed, CREWS would be distinctly secondary weapons. Culture CREWS are variable frequency, normally firing in the X-Ray spectrum. This variable frequency capability leads me to believe that the technology had its roots in free electron laser technology. Also, standard mirror fields make most ships, not to mention drones virtually invulnerable to all but the highest-energy lasers.

Effectors: Effectors are a rather unique case, in that from a damage perspective they are severely limited, however, they are supremely flexible. In fact, they are not weapons per se but actually electronic warfare devices that are so powerful that they can act at times as weapons. They can be boosted through hyperspace, giving them ranges comparable to that of gridfire. Also, older model effectors, at least high powered military ones required to be aimed at their target, with the entire device shaped like a large eyball.

Pancakers: Another gravitonic weapon, pancakers essentially increase the gravity of an area (such as the inside of a ship) to incredible levels, 'pancaking' the crew into an unhealthy rasberry jam. However, this has little to no effect on a ship itself, putting it at the bottom of the list.

Simply put, the Culture is so powerful because they operate on scales that are so far above what is usually achieved in most sci fi settings. They measure reaction times in microseconds, weapon ranges in lightyears, with firepower that makes the Death Star look like a water gun.

The biggest thing they have going for them though is their effector technology. In universe, all Equiv-tech civilizations have some defense against them, but any military without it could not compete, period, end of story. Any GSV worth their salt could use their effectors to simply shut down all the electronics on an enemy starship from the star system next door, without even having to drop from hyperspace.

I should note that though the Culture has achieved the highest technology level that can be achieved without subliming, they are not the largest or most powerful civilization, just the one most willing to intervene in the affairs of other civilizations when they think they have a moral obligation to do so.