In my opinion, the following are in fact the strongest beings\warriors of all time (a top 10 list):

  1. Teresa (self explanitory).
  2. Priscilla (self explanitory).
  3. Dauful (Riful and Dauf fused together) - they were formidable against Priscilla, whose power exceeds that of the Abyssal's Ones and in spite of that they were troublesome. 
  4. Cassandra (repeadtedly agreed upon in the manga and evidently, stronger than the 2 other most powerful no. 1 [Hysteria and Roxxane] except for Teresa).
  5. Roxanne (kept up with Cassandra in battle for quite a long time).
  6. Miria (having keeping up with Hysteria as an Abyssal One is truly impressive and commendable).
  7. Hysteria (one of the 8th best no.1 warriors of all time).
  8. Isley - A powerful Abyssal One, strong enough to maim Luciela, infamous and feared throughtout the land for his might, strtegic and initative mind.
  9. Irene\Ilene (incredible prowess and analytical grandeurs - seen with developing the Quick Sword technique. Extremly knowledgable and astute).
  10. Galatea (brilliant Yoki perception, uncanny wisdom and adeptness. Her maturity and intelligence are at a high level, for example, she was mature and experienced enough to synchronize Clare's Yoki thus saving her from Riful's provocation to awaken).