Paladin has the worst defense without Clemency. Adressing the things you said:
passive mitigation: correct me if I'm wrong on the numbers but I'm pretty sure Paladins shield gives a 20% chance to block for 20% damage reduction. That's a passive mitigation of 4%. DRK has passive self heal to compensate, so does GNB, never played Warrior. The migigation is propably still better than that passive self heal but the difference is neglectable.
Sheltron vs TBN: TBN is so much more powerful, it's not even comparable. Sheltron gives you 16% damage reduction (20% from 100% block rate minus the 4% you already had from blocks) while TBN gives a 40k shield on roughly the same cooldown. Come on, it's not even close.
best Invul: fair enough, Paladin wins on that one but then again, it's on a 7 minute cooldown.
DRK only has TBN: doesn't DRK have access to the exact same defensive cooldowns as Paladin? Both have Rampart, Shadow Wall is the same as Sentinel, TBN is better than Sheltron and that's pretty much it already for Paladin (again, not counting Clemency). DRK also gets AD on top of that.
AD is only useful on big pulls: isn't that pretty much the entire point of your discussion though? You said DRK is crap on big pulls compared to Paladin because it doesn't have Clemency. DRK has AD instead and it's amazing on big pulls.
GNB is considered the worst in defense: nah Paladin is worse, although not by much (again, not counting Clemency). The passive self heal should be pretty close to Paladins 4% passive mitigation. GNB has pretty much the same defensive cooldowns (Sheltron being a bit stronger than HoS due to its lower Cooldown) and then gets two additional cooldowns on top of that for free, although they are both not that strong (Camouflage and Aurora, which practically functions the same as another mitigation tool). Paladin can throw in Divine Veil, GNB throws in Heart of Light. Hollowed Ground is the only upside here and even then, Superbolide isn't that far behind it, in my opinion the strongest invul after HG.