Are my mitigation numbers off ?

[Lyrica_Ashtine]

Yes, it's not really 16 stacks, but you could use one CD for the missing stack every minute. And you can use Inner Beast without breaking your combo, meaning you can probably build one stack immediatly after Inner Beast.
And no, Fracture, is not included since the focus is mitigation. Again, if PLD is supposed to be the best, WAR shouldn't be able to top it, even by sacrificing some DPS.

Using Vengeance out of the blue to get a stack back is far from practical, thus that leaves only Berserk. Even if you'd decide to use both to get stacks, disregarding mitigating any tank busters that will come sooner or later, you'll still lose two GCDs out of 24 on IB/Fell Cleave. Considering you're not even taking Oaths, Grit or Defiance in consideration, you might as well just assume the stacks are used on Fell Cleave instead. Which has no mitigation potential at all.

Since you're still not considering Oaths/Grit, I'll do the numbers here for you. To start with, here are the ones you have without any other modifiers:

PLD:

• Rampart : 20*(20/90) = 4.44%
• Sentinel : 40*(10/180) = 2.22%
• Bulwark : (Let's be generous and consider 100% Block rate for the 15s) 30*(15/180) = 2.5
• Sheltron : 30*(3/30) = 3% (3s is close enough to cover "one hit")
• Rage Of Halone : 10% (I thinks its a little lower, but can't find the proper number)
• Hallowed Ground : 100*(10/420) = 2.38%

Total active mitigation : 24,55%

WAR:

• Storm's Path : 10%
• Vengeance : 30*(15/120) = 3.75%
• Inner Beast : 20*(6/15) = 8% (With a GCD on 2.5 seconds, you can do 24 GCD during each Infuriate, so 8 combos. Without any additionnal CD, 8 combos gives you 16 stacks of Wrath, so at least 3 Inner Beast + 1 from Infuriate. So one each 15s on average)
• Raw Intuition : 20*(20/90) = 4.44%
• Since Holmgang is not really a "mitigation" skill, I won't inclue it

Total active mitigation : 26,19%

DRK:

• Shadowskin : 20*(20/90) = 4.44%
• Shadow Wall : 30*(10/180) = 1.67%
• Reprisal : (Since Dark Dance has a 60s recast, and you can at least have one parry on its duration, I'll count one Reprisal per minute) 10*(20/60) = 3.33%
• Dark Dance : (A bit tricky, but with a proper set, you can probably parry half the hits during its duration, so, around 10s of mitigation) 20*(10/60) = 3.33%
• Dark Mind : (All with Dark Arts, since mitigation is the main goal) 30*(10/60) = 5%
• Delirium : 10% (See Rage Of Halone above)
• Walking Dead : (See Holmgang)

Total active mitigation : 27,78%

I won't use the exact same list as some effects can be on nearly permanently and not all mitigation types affect the same attacks:

Paladin
Shield Oath and Rage of Halone included.
Rampart: 36% magic (8%), 42.4% physical (9.42%)
Sentinel: 52% magic (2.89%), 56.8% physical (3.16%)
Bulwark: 0% magic (0%), 49.6% physical (4.13%)
Sheltron: 0% magic (0%), 49.6% physical (4.96%)
Hallowed Ground: 100% magic (2.38%) 100% physical (2.38%)
Active magic: 10.89%
Active physical: 21.67%

Warrior
Vengeance: 28% magic (3.5%) 28% physical (3.5%)
Inner Beast: 19% magic (7.6%) 19% physical (3.5%)
Raw Intuition: 0% magic (0%) 28% physical (6.22%)
Foresight: 0% magic (0%) 17.2% physical (2.15%)*
Active magic: 11.1%
Active physical: 19.47%

*I think it was 8%, someone correct me on that if it's wrong

Dark Knight
Shadowskin: 42.4% magic (9.42%) 36% physical (8%)
Shadow wall: 49.6% magic (2.76%) 44% physical (2.44%)
Reprisal: 35.2% magic (11.73%) 28% physical (9.33%)**
Dark Dance: 0% magic (0%) 36% physical (6%)
Dark Mind: 49.6% magic (8.27%) 0% physical (0%)
Active magic: 32.18%
Active physical: 25.78%

** It's impossible to use Reprisal twice/minute on average, but I'll humor you here.

Tally:
Magic
Dark Knight 32.18%
Paladin 13.27%
Warrior 11.1%

Physical
Dark Knight 25.78%
Paladin 24.05%
Warrior 19.47%

Now that I've finished this pointless task, take this with a grain of salt because:
2 Reprisal/minute is impossible
Convalescence isn't even taken in consideration (also: Mantra)
Awareness isn't taken in consideration
All stacks are used for Inner Beast, not a single one used on Fell Cleave
Storm's path counts for both tanks when the warrior is off-tanking, but not considered in the list
Self healing isn't taken in consideration (IB, Bloodbath, Storm's path, Souleater, Clemency)
Passive mitigation from blocking/parrying isn't taken in consideration
Performance loss for mitigation isn't considered

Performance loss? Yes, performance loss. Warriors lose mitigation ability when they use their stacks on Fell Cleave. Their "active mitigation" drops significantly when it is not spend on Inner beast on anything but tank busters - Which is perfectly fine and what they should do, in my opinion.
Using Dark Arts on anything but Soul Eater or Carve and Spit reduces the Dark Knight's damage output. Using it every 60 seconds possible on Dark Mind eats away on it significantly and the amount of tank busters that requires Dark Mind to be used that frequently is rare.
While Dark Mind is still the subject: Dark Mind does not work on nearly all auto attacks. The only exceptions I can think of at the moment are Ramuh and Manipulator.
Staying in Grit for an extended duration denies the use of Blood Weapon for Dark Knight.

Paladin CDs are pretty much "press this and do that". The major annoyance would be their ability to change oaths, which eats away on a GCD unless they simply toggle off Shield Oath and not go into Sword Oath like Dark Knights do (although that's a bit senseless). Warriors, while they have it easier to switch stances, are stuck in their stances for at least 15 seconds upon switching.

tldr; There are an impossible amount of factors involved. The most important one would be their output potential in terms of damage which Paladin is undeniably lacking compared to the other tanks. But in terms of effective active mitigation, paladins have it the easiest as it's a single button action without the use of resources.