[==Me==]

Excellent breakdown, though I think you mistook the Lord of Change for the Keeper of Secrets

I'll put ==My== thoughts in a similar fashion. I'll approach each unit on its own, then rank them in terms of their FoC slot.

Khorne
Bloodthirster: Very expensive, very killy. The BT costs as much as a Land Raider and can dish out some serious pain, but he is very expensive for what he does and doesn't even have S8 unless he charges or you pay for Unholy Might.
Herald: Relatively cheap killing machine. A chariot gives him the same stats as a Daemon Prince with 1 less WS and a 3+ save for cheaper! Add in Unholy Might and you have a very nasty HQ choice for only 100 points!
Bloodcrushers: Resilient, relatively inexpensive, and awesome in combat. Their only downside is a crippling lack of speed.
Bloodletters: 3 WS5 S5 I5 power weapon attacks on the charge? Yes please! A great Troops choice, only let down by their average speed and resilience (T4 and a 5+ invulnerable will only take you so far).
Flesh Hounds: The only Khorne unit that doesn't ignore armor, but it makes up for it with lots of attacks and crazy speed. They are ideal for tearing up weaker infantry (anything without a 3+ save) and holding down tougher units for the power weapons to chop them up.
Daemon Prince: 1 extra attack, not very many useful upgrades, and still only T5 with a 5+ invulnerable. Running helps a lot, but I think I'll pass.

Tzeentch
Lord of Change: Excellent shooting, very resilient, high speed, and a solid performer in combat. He ends up being the most expensive with all his useful gifts, but he can wipe out whole units every turn between his BS5, Breat of Chaos, Bolt of Tzeentch, Daemonic Gaze, and 4 attacks at S6 ignoring saves.
Herald: 50 points for a BS4 AP3 heavy bolter? Awesome. Add in a relatively cheap anti-tank power, the potential to fire both and at seperate targets, nifty abilities like Breath and Boon and the awesome chariot option makes him a very strong shooting HQ.
Flamers: Flying Breath of Chaos destroys just about anything, though 35 points for a single wound T4 model is a tad steep. A great support unit, but very much a one-hit wonder.
Horrors: Our most expensive Troops and our only shooters. Warpfire is a great ability, laying waste to everything short of marines and with plenty of shots to force enough saves on them. T3 is pretty soft, but a 4+ invulnerable makes them a lot more survivable. Also the only way to hurt tanks in Troops slots short of hoping for your BLs or Daemonettes to get lucky.
Screamers: Melta bombs are nice, flying melta bombs that can turbo boost are even better. They are ideal tank hunters, though they are inept at anything else and make for easy kill points. I would take them if I'm light on anti-tank or have points to spare, and then I'd only take 3-4. 48 points for 3 flying melta bombs is great, it won't even hurt that much if your enemy doesn't field tanks.
Daemon Prince: A great shooter with an even greater cost associated with it. The 4+ invulnerable is a big improvement, but T5 still hurts. The DP sacrifices the speed and gifts of the LoC for better abilities in combat and a lower cost.

Nurgle
Great Unclean One: T6, 4+ invulnerable, FNP, and plenty of wounds balanced out by S&P, few attacks, low I and low WS. The GUO won't die, but he won't be doing as much damage as any other GD. However, it is the cheapest GD out there and still very deadly. Noxious Touch makes those few attacks much more useful, and upgrades like Cloud of Flies and Breath of Chaos enhance his abilities and make him much more threatening while not increasing his cost by much.
Herald: Bleh. A Plaguebearer with an extra wound and attack for the price of 3 precious, scoring Plaguebearers. With Breath and Cloud he could be useful, but he is nothing compared to Epidemius or any other Herald.
Beast of Nurgle: A Plaguebearer with random attacks and an extra wound for the cost of 3. Not a bad choice, but other Elites are more useful and in a purely Nurgle list you're better off with more Plaguebearers, Daemon Princes, and Nurglings.
Plaguebearers: These guys don't die. Ever. They are slow as molasses and couldn't fight their way out of a wet paper bag, but they are so tough it won't matter. Ideal for holding objectives and icons but little else. With Epidemius in the mix, however, they can be quite nasty.
Nurglings: Their only downside is not scoring. Our cheapest Troops, Nurglings can't be instakilled, don't suffer from S&P, and have plenty of wounds. Ideal tarpit unit, and downright nasty in an Epidemius list.
Daemon Prince: One of our better DP builds. T6 makes all the difference, and awesome gifts like Noxious Touch and Cloud of Flies sweeten the deal.

Slaanesh
Keeper of Secrets: A great GD, probably the best. S6 T6, Fleet, lots of attacks, and a ridiculously high I make the Keeper an awesome choice. Slap on Transifixing Gaze and Sopoforic Musk and you have a true beast.
Herald: A tad on the soft side and rending isn't all that deadly now. However, they are relatively inexpensive and their mount options are quite attractive. Very fast, potentially deadly, but fragile.
Fiend of Slaanesh: Fast, lots of rending attacks, H&R, high I, and a solid S and T. An exceptional choice for roughly twice the cost of a Daemonette and easily 10 times the worth.
Daemonettes: Very fragile, lots of rending attacks, high I, and fleet. They would be nice if it weren't for Seekers being so much better for 3 points more. Running makes fleet not nearly as good, rending isn't nearly as threatening, and T3 just makes these girls bolter bait. It wouldn't be so bad if they didn't cost a fair bit of points for a decent-sized squad. Transifixing gaze is a nice and cheap upgrade though, always worth it. I would only take these if running a pure Slaanesh list and I was forced to.
Seekers: Daemonettes that move as cavalry with an extra attack, sign ==Me== up! One of our best Fast Attack choices, their fragility is made up for by their speed and bucketload of attacks.
Daemon Prince: Extra I, whoop dee doo. T5 and a 5+ inv hurts them a lot. However, awesome gifts like Aquiescence, Musk, Pavane, and T. Gaze make them useful. Be a little more selective with their targets and you'll be fine.

Undivided
Furies: Horrible. Worst unit in the Codex by far (only eking out past the Nurgle Herald). Take Flesh Hounds for the points any day.
Soul Grinder: A cheap AV13 walker with tons of nasty shooting and a solid HtH performance. Phlegm ruins anything short of Terminators, Tongue is only BS3 but it's a freakin' railgun (huge for an army chronically lacking in anti-tank firepower), and Vomit decimates hordes. Plus the harvester gun is a very nasty secondary weapon. You low your maw cannon? Big deal, you have Fleet and 5 S10 attacks charging. A real competitor to the Daemon Prince and probably one of the best units in the Codex. It's only downside is that it takes up a Heavy slot and is out only vehicle.

That's what I've come up with based on ==My== playtesting and theoryhammering. Now here's how they all stack up in their respective FOC slots.

HQ
Greater Daemons:
1) Keeper (cheap, exceptionally killy, and fast)
2) Lord of Change (great firepower)
3) GUO (extremely tough, cheapest GD by 40 points and no slouch with the right gifts)
4) Bloodthirster (too expensive for too little)
Heralds:
1) Khorne (dirt cheap for what it can do)
2) Tzeentch (cheap firepower at a good BS and potential high speed)
3) Slaanesh (soft, but very speedy and lots of attacks)
7) Nurgle (awful, just awful)

Elites (very hotly contested slot, everything here is more subjective than anything else)
1) Fiends (fast, cheap, great in combat)
2) Bloodcrushers (deadly in HtH, hard to kill, but slow)
3) Flamers (capable of wiping out entire units of Terminators, very fast and good BS, but expensive and fragile)
7) Beast (just not killy enough for its points, also slow)

Troops
1) Bloodletters (extremely good in combat, fairly tough and not too expensive)
2) Plaguebearers (super tough, ideal for objectives and icons, but slow and feeble in HtH)
3) Horrors (great shooting, our only anti-tank, and inv save, but T3, BS3, high cost, and horrible HtH abilities bring them down here)
4) Nurglings (they are only here because they don't score)
5) Daemonettes (too fragile, not fast or killy enough to stand out)

Fast Attack
1) Flesh Hounds (fast, relatively tough, and hit hard in combat)
2) Seekers (not as tough as dogs, but plenty fast and lots of rending)
2) Screamers (Great for tank-hunting, speedy, and cheap if a bit fragile. They don't really compare to dogs and seekers, but are a good choice anyway)
11) Furies (suck given form)

Heavy Support (yes it is a tie, they're both exceptional)
1) Soul Grinder (extremely versatile, low cost, and a high AV)
1) Daemon Prince (very mutable, another multi-wound MC, and extremely good at whatever role or roles you give it)

Hope that helps, apologies for the length. Also bear in mind that this was written in terms of mixed lists. Monolists will obviously play differently. Also, keep in mind this was written in terms of ==My== playtest experiences and some theory-hammer. Purely opinion.