Here is my vision of experience ranks:
Hell Knight
* Kill 500 hell knights.
* Kill 100 hell knights with a pistol.
* Kill 100 hell knights with a combat knife.
Hell Baron
* Kill 700 barons of hell.
* Kill 2000 enemies with a shotgun (any)
* Kill 10000 enemies.
Arachnotron
* Kill 1000 arachnotrons.
* Kill 2000 enemies with a plasma rifle.
* Kill 250 arachnotrons with a plasma rifle.
Mancubus
* Kill 400 mancubi.
* Kill 500(800?) enemies with a RL.
* Kill 80 mancubi in melee.
Arch-Vile
* Kill 700 Arch-Viles
* Kill 2000 enemies in melee
* Kill 500 enemies with a BFG
Cyberdemon
* Kill 100 Cyberdemons
* Kill 40000 enemies
* Kill 100 enemies with a Dragonslayer (Mwa-ha-ha-ha!)
The point is to make player use all the weapons in the game. They are "experience" ranks, after all.
For challenges - i'd place them in such order, by ascension of difficulty:
AoD
AoMr
AoH
AoRA
AoMC
Ao100
AoPw
AoD&D
AoLT
AoP
AoI
AoB
AoMs
Challenge Difficulty IMO (Ones that are more gimmicky than actually hard left out)
Ao100
AoMs
AoB
AoPu
AoPa
AoI
AoLT
AoRA
AoMr
As such, here are my ranks. Spoiled for huge.
Cannon Fodder:
No requirements.
Private:
Reach level 10.
Unlocks AoLT, AoMr, AoD, HMP.
Corporal:
Reach level 15.
Reach level 10 on AoLT.
Kill an Arch-Vile.
Unlocks AoI, AoDD, AoPo, UV.
Sergeant:
Win!
Reach level 10 on AoI.
Reach level 15 on AoLT.
Unlocks AoB, AoRA.
Lieutenant:
Win AoLT.
Reach level 15 on AoI.
Reach level 10 on AoB.
Unlocks AoPu, AoH, AoMC, Nightmare!
Captain:
Win HMP.
Win AoI.
Win AoPu.
Unlocks everything.
Major:
Win UV.
Win AoB.
Win AoMs.
Colonel:
Win Nightmare!
General:
Win AoLT on Nightmare!
Win AoMs on HMP.
Marshal:
Win AoI on Nightmare!
Win AoMs on UV.
Blade:
Win AoPu on Nightmare!
Experience Ranks, spoiled for huge.
Each rank takes exactly double the time as the previous, according to my kill list. I pefer melee games more than the average joe, so I've toned down the melee stats.
The starting rank, kill 50 former humans, is a 200th of how much I play (9948 formers and counting). As such, the 8th rank is where someone who's been playing as long as I have (a lot.) will be.
Human:
No Requirement.
Former Human (200th of my stats):
Kill 50 former humans.
Imp (100th):
Kill 100 imps.
Die 10 times. [Hang in there, champ!]
(I've only died 300 times, but 3 seems too low.)
Demon (50th):
Kill 100 demons.
Kill 100 things in melee.
Kill 10 demons in melee.
Cacodemon (25th):
Kill 100 cacodemons.
Kill 10 cacodemons with a pistol.
Kill 750 things on the "Hurt Me Plenty" difficulty, or higher.
Mancubus (10th):
Kill 20 mancubi (Yeah, I've only got 200.)
Kill 5 mancubi with pistol or melee.
Kill 5000 things.
Hell Knight (fifth)
Kill 200 hell knights.
Kill 20 hell knights in melee (I have 100! I'm just as surprised as you.)
Kill 50 of everything, barring bosses.
Revenant (half)
Kill 100 revenants, and hate them with half the fibers of your being.
Kill 10000 things on the "Hurt Me Plenty" difficulty or harder.
Kill 8000 former foos.
Hell Baron (One-th?)
Kill 600 Barons of Hell. (100 of them were in melee! What was I thinking!?)
Kill 50 Barons with the pistol.
Kill 2000 things in melee. (Okay I really have double this)
Arch-Vile (50% more than me, double would take years)
Kill 300 Viles.
Kill 5000 things with the pistol.
Kill 25000 things on the "Hurt Me Plenty" difficulty or harder.
Cyberdemon (Double me)
Kill 50 cyberdemons.
Kill 10000 things in melee.
Kill 75000 things.