+ Stoneskin is by far the best PLD Cross Class Skill for reasons already stated above. You can increase your own survivability drastically with it, especially when casts are properly timed.
+ Foresight - the only other cross-class mitigation tool you can get. Pretty weak by itself versus one big hit, but adds up over time. I pop it alongside Awareness and Convalescence.
+ Bloodbath - can be effective at higher levels, even for PLD. Extra Undebuffable Healing.
+ Mercy Stroke - serves two purposes. (i) Allows you to tell when an enemy crosses a certain HP threshold (ii) Extra Spike damage, useful on Conflags and Gaols. Never rely on the heal when teamed.

For the 5th? Usually Cure or Protect.
Stoneskin beats Cure in nearly all possible endgame situations.... but there's a few points in tough fights where a Healer or SMN has enough time to raise someone but not to protect them.
I'd take Raise if you could cast it in-combat, but you can just skillswap outside combat anyway when needed.