This is power at an individual level. It means that it would be possible for all the endless if they fought together to beat Gabriel Hornblower. There is also the fact that cleverness and luck play big parts in who wins a fight. Serafall is stronger than Odin but it doesn't mean that she would win against him. Rias isn't even even in the top hundred at least for now
1/ Presence( Abrahamic god)/ Darkness/ Father time
2/ Michael Demiurgos( full power)/ Lucifer Morningstar
3/ Gabriel Hornblower
4/ Death of the endless
5/Destiny of the Endless/ Dream of the Endless ( big red)
6/ Destruction of the endless (Olethros)/ Ophis
7/The others Endless like Delirium, Desire and Despair
8/ Sirzechs Gremory/ Ajuka Astaroth
8/ Brahma/ Shiva/ Vishnu
9/ Indra/ Rizevim Lucifer
10/ Ddraig/Albion
11/ Nyx
12/Gaia/ Tartarus
13/ Michael Demiurgos depowered
14/ Seraphs like Azazel, Kokabiel and Uriel/ Archdemons like Bael, the original Sitri, the Original Belial and the original Satans
15/ ten winged angels/ Demons like the original Gremory/ Archdevils like Serafall
16/ Kronos before being jumped by his children
17/ Crom cruach
18/ Hades
19/ Lugh
20/ Siddhartha Gautama