1. Vampires and other undead: do not age, are generally immune to disease, can survive superhuman levels of damage, but can be permanently destroyed by fire, sunlight, holy power, running water, exorcism, or similar means depending on the setting. This includes the Scarlet sisters and the Hakugyokoro faction.
2. Deities: exists outside of the mortal coil, cannot be permanently destroyed in most settings. This includes the Higan faction, Suwako and Kanako, etc.
3. Terrasques: will regenerate from any level of physical destruction, including nuclear annihilation. Can only be eliminated permanently by sufficiently damaging the body and then using a Wish spell to make the creature have never existed in the first place. Mokou and Kaguya would fall in this category, as well as all of the faeries of Gensokyo.
So logically, terrasques are the strongest terrestrial creatures in the world. Faeries are equivalent to terrasques. Cirno is a faerie, so she is equal to a terrasque. Therefore Cirno is the strongest, QED.
So....?