What armies best represent the upper tier of power level?

If uncomped, and purely competitive, I'd list them as follows...

Tier 1
Dark Elves
Warriors of Chaos
Daemons of Chaos
Skaven

Tier 2
Vampire Counts
Lizardmen
Chaos Dwarfs
Wood Elves

Tier 3
High Elves
Ogre Kingdoms
Empire
Dwarfs

Tier 4
Orcs & Goblins
Bretonnia
Beastmen
Tomb Kings

Tiers 2 & 3 could be 1 tier, I just split them up for my personal ranking.

Warriors of Chaos are definitely in the top tier - a lot of my games when learning WFB were against them (and very painful, even though I was playing the 7th ed DE book back then), I then had a friend who loved the unkillable Nurgle Daemon Prince for a long time, and in the last few years I've played WoC myself - I think WoC have been involved in more of my games than any other army, so I get that they're potentially extremely hard to play against.

However, I don't get the emphasis on the basic Chaos Warriors themselves. Yes, model for model, each is incredibly good. However, that's reflected in their points cost - they might be as good as an equivalent Rare choice, but they cost at least the same, which is why you rarely see more than 18 in a unit. They're not all that fast, their leadership isn't spectacular, and actually within a WoC army they're one of the easier aspects to address.

It's the Daemon Princes (with pet Chimeras), Disclords, Gorebeast Chariots, Skullcrushers and so on that make WoC so nasty - tough, extremely killy, and highly mobile, making them much harder to counter.

I think I also need to play Knoffles with a DE army, so he can see they're more than just WE hordes/Peg masters/dual Hydras (try triple Hydras )

I believe the main problem with Chaos Warriors as they are is that even with the points cost, they are too good to be a core unit - the fact that WoC players can spam such a strong close combat unit to their heart's content (indeed they are forced to take at least 25% of their points' worth of them) is what makes them OP. 

I mean, it would be easy to make them less OP but also show the Chaos Warriors' elite status - have a look at the following profile below:

M    WS    BS    S    T    W    I    A    Ld
4      4       4     4    4    1    4    1     8

The main change I've made is reduced their attacks to 1 minimum - they're humanoids without any features other than a hand weapon to attack with, so they should by all accounts have 1 attack. Note that they can still be given extra hand weapons and Mark of Khorne to increase their maximum number of attacks to 3 each - I think this is plenty for a core unit.

I've also reduced their weapon skill to 4, as 4 is still the same weapon skill of Dwarfs and Elves, which is still far superior to a man of the Empire or Skaven Clanrat. Of course I also think that Marauders should also get this Weapon Skill penalty so that they're weapon skill 3, as we can't have full-blown Chaos Warriors not being more skilled than a lowly Marauder, but at the same time the Marauders are simple barbarian warriors - sure they're rough tough fellows who are handy with an axe, but would a simple barbarian like a Marauder be as skilled as a Dwarf or Elf, the former probably having trained for around 20-30 years, the latter ten times that (assuming Warhammer Elves are immortal like Middle Earth Elves)? I don't think so, especially as Marauders are little more than Ancient Germans with a Chaos taint and a Conan-type aesthetic.

I've also reduced their initiative to 4, which still doesn't do much to halt the advantage they have over Dwarfs and Ogres, but at least it gives Elf spearmen more of a chance to do something before being hacked to death. In any case, would a fully-armoured Chaos Warrior be as athletic as an Elf? Certainly not. Firstly they've got the weight of their super-heavy armour to contend with, and secondly they're more likely mutated humans than mutated Elves, and so they've got relative human slowness - after all they are Movement 4 rather than Movement 5.

Of course this will require a slight points reduction - I imagine around 12 pts per model would work.

One final thing is that if the Great Weapon option was removed from Warriors, this would give Chosen more viability, especially if they kept the original Chaos Warrior profile - then they would be actual elite troops rather than a more expensive Chaos Warrior with great weapon. Also if anyone already has Great Weapon warriors, there's nothing to stop them saying that they are Chosen if these profile changes come round - they look practically identical as it is.

I don't think this solves everything about WoC being OP, but I like to think it's a nudge in the right direction.

I've only made 3 votes currently - WoC, High Elves and Ogres. I know many say that Ogres aren't that OP, but personally I just haven't found a way to crack them yet. If I do find a way, I might change my vote.

Conversely many say that Dark Elves are OP, but I haven't actually played them and don't know many of their rules so I can't say for sure. I do know that they were one of the armies that unfairly got two White Dwarfs' worth of miniatures rather than the single month's release like Warriors of Chaos (and also Vampire Counts) and if it wasn't for GW's favouritism towards them one of Bretonnia or Beastmen could have got a book and an update, but I don't think I can make a proper judgment without reading their army book and rules.

Didn't know that - Dark Elves were first updated in September 2013 and got a second wave the following month, while End Times was the following year. That actually seems surprising to me because the Witch Elves and Cauldron of Blood both appeared in September rather than October - Doomfire Warlocks definitely came out in October and possibly Dreadspears too. 

However, Vampire Counts and WoC still both got 2 even before End Times (Vampire Counts got releases in August 2011 and January 2012, the latter being when they got their updated book, and WoC got releases in November 2012 and January 2013, again the latter being when they got their book). Two of those months could easily have been used for Bretonnia and Beastmen - GW were just too spineless to give them the attention they deserved.

lordofskullpass I think the WS on marauders and warriors is ok. Marauders are meant to all be seasoned warriors that live for fighting from a young age (yes I’m reading the wulfrik books at the minute 😉) so to give them the same WS as a lowly standard human seems wrong and warriors are elites so their WS matches most others armies elites. The current WS then nicely follows onto the characters WS. A basic Warriors is 14pts a model. That is the same as a hammerer. Yes one is core vs special but bar initiative the stats are fairly similar (hammerers get great weapons (so +2S & ASL, -1 save, stubborn and +1 LD). After upgrading your warriors they are likely to be at least 17pts (2pts for the marks and at least a Shield).
I think the main issue with warriors core is that the marauders cost too much in comparison to a warrior (especially if kitted our reasonably). If they were cheaper then you would definitely see them fielded more often. I believe this is a direct result of rules overcompensation from the previous edition.

I don't think that WoC deserve the hate that they're getting. They've ALWAYS been an army of elite psychopaths who wander down from the north to maim burn and kill and then once they're inevitably stopped, the survivors return back home to brood and plot their next attempt.

Warriors have great stats sure, but you just can't look at them compared to what's in other books, you have to look at them compared to what's in THEIR book. Marauders are better than your average human (I'm sure they learn to fight and kill long before "normal" humans do hence their better than normal human stats) and Warriors are elite compared to them.

Something that's often overlooked - Warriors in the second and beyond ranks are paying for an attack that they don't get to use because of the way supporting attacks work. They're actually not nearly as "points efficient" as other units with only a single attack from other army books.

lordofskullpass "I've also reduced their initiative to 4, which still doesn't do much to halt the advantage they have over Dwarfs and Ogres, but at least it gives Elf spearmen more of a chance to do something before being hacked to death. In any case, would a fully-armoured Chaos Warrior be as athletic as an Elf? Certainly not. Firstly they've got the weight of their super-heavy armour to contend with, and secondly they're more likely mutated humans than mutated Elves, and so they've got relative human slowness - after all they are Movement 4 rather than Movement 5."

Elves still get to swing first, with their rerolls. If your Initiative is EQUAL TO OR GREATER than that of your foe then you get your rerolls. If you're going to arbitrarily nerf Chaos Warriors due to these "real life reasons" than you need to do the same thing to Empire/Bretonnian characters and units. If there's no reason that these frenzied barbarians from the north who fight and war amongst each other and neighboring kingdoms all the time can't be justified as having WS4 because of the greater races such as dwarves and elves, then there's no reason that an empire swordsman, or a greatsword, or a knight should have that WS4 either. And that I5 that those Empire/Bretonnian characters rock? Guess that's unbelievable in the grand schema of dwarf and elf stats too so might as well nerf them.

While we're talking about Elves - can we please talk about how amazing it is that they get to reroll their hits EVERY SINGLE ROUND OF COMBAT? Warriors of Chaos are ONE bad combat away (unlucky dice) from breaking and being run-down; they almost NEVER have the numbers to be/break steadfast - they have to rely on their great stats and armor to wear down their foes over multiple turns of combat. Elves however almost NEVER have to worry about a bad round of combat as their innate special rules keep them consistently outputting average damages.