I'm usually asked alot in game as to what guard I should do and all this and that. So I figured it would be easier to make a thread to be able to resort back to for questions. Now these ideas are based on MY opinion as I have just about all the useful possible guards on one toon and can really determine what seems to work where and when.
Melee type build (including tanks with no ac): Non epic
1st- Disintegrate 13.5 untyped damage per hit
2nd- Lightning Strike 12 electric damage per hit
3rd- Corrosive Salt 8.3 untyped damage per hit
4th- Steam 4.5 untyped damage per hit
5th- Crushing Wave 4.86 bludgeoning and 4.86 cold damage per hit
6th- Incineration 14.5 fire damage per hit
The damage per hit is taken in account the approximate amount of damage a guard will deal divided by the amount it procs in 100 hits. Incineration will deal the most overall damage but as end game is all immune to fire I put it down as the 6th top DPS guard to have.
Melee type build (including tanks with no ac): Epic
1st- Freezing Ice 5% on 35 fort DC to auto-crit for 8 seconds
2nd- Earth Grab 5% on 35 reflex DC to auto-crit for 6 seconds
3rd- Trap the soul 2% on 30 will DC to auto-kill
4th- Incineration (see above) highest possible DPS guard
5th- Disintegration (see above) 2nd highest
6th- Lightning Strike (see above) 3rd highest
Couple things to note here DO NOT use radiance. Mobs still seem to move around like crazy when they are blinded and hit with a mass hold, or dance or whatever the case. So please put your radiance guard down here. I'm also certain air guard does the same thing so please leave these two guards at home.
Melee type build with AC (AC relevant enough for end-game excluding epics)
1st- Radiance 20% to deal 4d6 light damage + blind
2nd- Steam 25% to deal 2d8+8 untyped
3rd- Earthen not sure the exact proc rate but I know it's high and casts stonekin
4th- Enervation ~5% to drain negative levels
5th- Freezing Ice/Earth Grab
6th- Incineration
I know it's tough to consider wearing guards with an AC toon so these are some of the guards I figured will help the best. Most of them are impacting your AC directly by affecting a mob. Whether it's making them blind, giving them negative levels or auto-criting them. AC builds also do get hit with glancing blows so the damage portion of radiance and then your steam here are big with their high proc rates. They might not do much damage but with raid boss tanking this adds up.
Ranged DPS builds
1st- Freezing Ice Auto crit
2nd- Earth Grab Auto crit
3rd- Air guard 2.5 chance to knockdown on a 30 reflex save
4th- Radiance Auto blind
5th- Magma Surge 5.23 fire damage per hit with a 6 second slow effect
6th- Slay Living 5% on 30 will DC to have slay living casted
Again this goes back to the idea of NOT being hit as you're ranged DPS but these guards are here to help you maintain just that if you are being hit. All of these guards effect movement with the last one giving you the chance of just killing your mob out right.
Raid boss tank
1st- Disintegrate
2nd- Lightning Strike
3rd- Corrosive Salt
4th- Steam
5th- Crushing Wave
6th- Earthen Guard
A raid boss tank's ideal when they are minimzing damage taken with maximizing damage out put so these are the type of guards that you'd want to look into if your character is going to be fullfiling that role in your raid.
So this is my guide to building your guards based on your toon build and what not. Again this is all based on my opinions in all these situations with the guards on one character. If you don't agree great do your thing. This is just a reference guide for those that are interested and having questions concerning what I should do when. Hope it helps you.