This tier list/ranking goes off of in game abilities and known lore. Agents within each respective tier are only ordered alphabetically; the lore is a bit too vague for me to rank them from weakest to strongest within their tier.
The 4 tiers are: human (no powers, standard human), radiant (superpowered individual with powerful abilities), powerful radiant (superpowered individuals with incredibly powerful abilities superior to other radiants) and KAY/O (I will explain why he has his own tier when we get to it.)

Human
These agents are normal humans with no supernatural abilities.

Breach
Brimstone
Chamber
Cypher
Killjoy
Raze
Sova
Viper

Radiant
These agents are superpowered individuals with powerful abilities. Some radiants don't have very impressive abilities, however others are pretty dangerous capable of altering nature itself.

Jett (Ability to control the wind to levitate objects and move quickly through the air)
Omen (Ability to teleport across large distances, conjure smoke clouds and manipulate minds)
Phoenix (Ability to manipulate fire, capable of temporary immortality)
Reyna (Ability to consume souls, which makes her physically stronger)
Sage (Ability to manipulate radianite itself to form walls and slow fields, to heal and to resurrect)
Skye (Ability to bring wooden figures to life and control them; these can heavily weaken and disrupt)

Powerful Radiant
Two radiants stand out as more powerful than the rest, and that is because they can ignore and even alter space and reality to their will.

Astra (Can harness the power of the cosmos to enter an astral form, where she can alter reality on a massive scale, capable of creating micro black holes, nebulas, nova pulses, and indefinite walls of cosmic energy)

Yoru (Ability to rift walk and traverse other realities, timelines and dimensions. Can use this to teleport, go invisible to the human eye, conjure blinding charges and tethers)

KAY/O
The reason KAY/O is at the top of our list and has his own tier, is because he has an incredibly overpowered ability that literally prevents radiants from using their power and humans from using their radianite technology, therefore he hard counters every other agent. Theoretically, he could easily beat somebody as powerful as Astra because although she is technically more powerful, he can stop her from using her power altogether, making her just human.

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Let me know if you guys want me to rank every agent from weakest to strongest rather than an unordered tier list, although the lore isn't detailed enough for me to do this fairly, I could use known information to create a ranking based off of what we know of their powers/abilities.