1. Stoneskin - Far and away the absolutely most useful PLD cross class hands down. You can help healers when DPS are taking damage, you can drain your entire mana pool to give yourself a CD-like ability to mitigate damage by self casting, you can assist in recovery, you can cast it on your WAR friends for huge amounts of mitigation while OTing, etc. It is amazing and adds a nice little layer of depth to PLD.
2. Foresight - Another defensive CD, no matter how crappy it is, I usually pop with along with Awareness/Bulwark to combine those 3 into a passable CD (I know Bulwark is OK by itself, but those 3 together make using them a little more impactful)
3. Bloodbath - I use this on CD hooked into my FoF/Flast Blade macro. Nice to start a fight with some self-overhealing and a miniscule amount of mitigation.
4. Cure - I like having cure in my pocket for silly things like if I am standing somewhere I shouldn't be and take DoT dmg, or something like CT when I am doing a tower and taking damage from the AE.
5. Mercy Stroke/Protect - We're getting to the bottom of the barrel here. Mercy stroke OK for extra DPS, and when you actually get the last hit in and get a 1k+ heal it feel pretty good! Protect I have used rarely, in using protect on the party when we have more than 1 member who has died and been raised during a tough fight.

There are all my personal opinions, been through T4 in coil with my PLD.