Tier 1: Immortal, God-like powers
Deities - Gwyndolin, Gwynevere, Gwyn (pre Second Kindling), Velka, Fina
Ancient Dragons - Seath, Stone Dragons, Kalameet
Gravelord - Nito
Tier 2: Immortal, magic powers but not on the level of the Gods
Angels - Witch of Izalith and her Daughters (pre Bed of Chaos), Gwyn (post Second Kindling), Artorias, Ornstein, Smough, Ciaran, Gough, Sif, Alvina, Big Hat Logan, Domhnall of Zena, Silver Knights, Black Knights
Crossbreeds - Priscilla
Demons - Ceaseless Discharge, Bed of Chaos, Quelaag (demon form), Quelaan (demon form), Demon enemies
Primordial Serpents - Kaathe, Frampt
Giants - Giant Blacksmith, Hawkeye Gough, Giant Knights
Modern Dragons - Hellkite, Drakes
Abyssals - Darkwraiths, Manus
Tier 3: Mortal, access to magic powers but not on the level of the Gods
Darksign Branded Humans - Player Character, Pygmy, Solaire (and other humanoid phantoms), Salaman, Laurientis (and other trainers), Andrei, Vamos
Hollows - Undead Merchants, other Hollows, Balder Knights, etc
Undead - Skeletons
Wildlife - Rats, Trees, Snakes, etc
Notes:
I use the word "Angels" for lack of a better term because they are:
1) Immortal
2) Possibly of divine origin
3) Lack humanity despite being sentient
4) Can ascend to godhood or regress as well (as with the case with Gwyn)
No one on the Tier 2 list of beings drop Humanity (except for Manus), that's the main qualifier. Otherwise they are a Tier 3 being. All bosses drop humanity except for the Four knights of Gwyn, and Gwyn himself.
Both Hawkeye Gough and Ciaran refer to the player character as "human", and Artorias as well if that quote list is anything to go by. This means that they are some race removed from humans but not quite gods. They also drop no Humanity, even Ciaran, though she looks perfectly fine and healthy.
In the prologue, when the voiceover talks about disparity, she's not talking about the rising of humans, but the creation of two separate races, Angels and Humans, Heat (divinity) and Cold (non-divinity), Life (immortality) and Death (mortality), Light and Dark. Since "Dark" is a recurring term when talking about humans (Manus, the Abyss, the Dark Soul), it only makes sense that Light probably refers to the divine beings. Which also explains why Gwyn and co want to extend the Age of Fire, which is a time of Gods, while the Pygmy and Kaathe want to usher in the Age of Dark (the age of Man).