This has come up a lot recently and I'm going to reiterate my statements from a few weeks back.
Elizes is not a survival increase. A defensive legendary + pulverize is more defense than elize + R&T.
The biggest winner is always chest, the damage taken bonus goes beyond any other legendary.
After that comes the ring for pure single target melee heavy fights. This is almost no really challenging fight in Tomb.
Goroth doesnt kill you with autoattacks, he kills you with meteor/star damage when you have the debuff.
Harjatan comes closest with his physical damage dot. But there's still the draw in frost damage to consider.
You can say something for every fight.
Maiden is straightforward, Avatar too. Kiljaeden is the only guy who will kill you with physical damage, but the real damage lies in fel claws which you use Bristling fur and/or cooldowns for. The extra mangle rage from ironfur won't really be a lifesaver, just a mana saver for your healers.
If the ring doesn't gain full value (a patchwerk/kiljaeden like fight), next in line are our heal/absorb legendaries.
Prydaz vs Skysec vs Archimonde's.
Prydaz and Skysec are competitive in HPS. If you are taking shitloads of damage skysec can heal for a lot when you want it to. Prydaz gives you an absorb you can't control. The absorb might be wasted half the time. But sometimes you don't really want control, just the absorb that increases your EH.
Archimonde's doesn't do near the HPS the two others do, but it gives you control (like skysecs) over an absorb.
If the control (skysecs) is what you need, but you won't survive the hit with your current HP pool (and thus can't heal it back up with skysec) Archimonde's is your go to. Then again, you could use the stamina/absorb trinket from TOS or Guldan in that case and still use skysec/prydaz.