Let's look at what relics are available to tanks:

Dread Guard Defense on click

Dread Guard Shield/Absorb on click

Dread Guard Absorb on Proc

War Hero static Defense

War Hero static Shield

Matrix Cube (Juggs and PTs only)

Before we talk about relics, let's ask what your actual chances are for defensing and shielding an attack is (if you theorycraft yourself skip to the next paragraph). With your current stats, your 27% Defense chance will be buffed up to 30% with Retaliation. Now the base accuracy for boss attacks is 90% aside from "special" attacks as the developers call it, couple that with your Smash debuff and the bosses accuracy is at 85%. Now the chance of dodging the attack is a roll of Defense against Accuracy, and basically is 100% - (85% - 30%), or 45%. Quite a bit larger than your listed Defense chance. The next roll is for Shields, all attacks that bypass your Defense roll have a 50% (rounded) chance of being shielded. The probability of and attack bypassing Defense and then being shielded is then .55*.5, or 27.5% chance to shield.

Now, to start crossing relics off the list. Dread Guard clicky relics share cooldowns, so you only want one. The Defense click relic gives better mitigation to a Jugg than a Shield/Absorb click, so the Shield/Absorb click is tossed out.

The War Hero Shield relic is not ideal for Juggs, you pick up around 550 Shield Rating just through Enhancements and Implants alone unless you're grabbing a lot of Accuracy (which you shouldn't). All calculations that take a total number of defense/shield/absorb points to be allocated, and finds the mathematically ideal allocation show that the amount of Shield rating you have to carry on enhancements and implants is the correct amount of Shield.

Now, the DG Absorb proc relic. You get the boost for 6 seconds after you shield. The slowest attacks will be around 1 attack every GCD. For 6 seconds you have 4 GCD's, so at worst 6 attacks. With our 27.5% chance to shield, it is expected to shield only 1 of those 4 attacks with the Proc still up. Furthermore, the 20 second rate will not be on a 20 second cooldown as there will be 2-3 GCD's after the 20 seconds ends before you shield again to proc it. Now at the upper end, the last phase of NiM Kephess is 5 attacks every GCD (bye bye Sin tanks). That leads to 20 attacks in the 6 second proc window, and at 27.5% chance to shield you expect to absorb 5.5 attacks, and have the 20 second rate constantly refreshed. Each of those attacks Kephess does hits for 2700 after armor. So at 50% absorb, a shielded hit is 1350 damage. Without the proc up, shielding 5.5 attacks in 6 seconds is 7425 damage taken, or 1238 DPS (just from the shielded attacks). With the proc up, you gain roughly 10% absorb, at which point a shielded attack hits for 1080, and shielding 5.5 attacks during the proc comes out to 5940 damage taken, or 990 DPS (just from the shielded attack). That's 248 less damage taken per second during the proc window, but factoring in the proc only being up 6 out of 20 seconds, it averages out to 74 less damage taken per second.

In comparison, the Matrix Tank Cube has a static +21 Defense chance on it. That adds about .4% Defense Chance. When looking at the tank melting numbers from Kephess (19500 DPS pre-mitigation) that made the absorb proc relic look viable, that .4% Defense Chance actually works out to 30 less damage taken per second. Not quite where the absorb proc is, but you have to realize that this is one phase of one fight. Every other fight in the game save the TFB tentacles attacks with far less frequency, at which point the RNG will go full 6 second windows where you don't shield again. For Hybrid Juggs with less than 45% shield chance, and those like myself with 31% Defense Chance, the math is even more in favor of a Matrix Cube providing all around more mitigation than the Absorb proc, and has the added bonus of a lot of Strength to help with threat.

The last relic to mention is a War Hero static Defense relic. There shouldn't be any argument about this one, it's a viable relic and the only knocks against it are being low endurance and without power or main stat. That leaves the DG Defense click, War Hero Defense, and Matrix Cube as the three more viable relics for appropriately geared Jugg tanks (if your defense chance is below 25% I can't help you...). Personally, I pick the DG and Matrix cube because I like the main stat boost, higher endurance, and less pvp involved with the Matrix Cube over the War Hero, but most combinations of those 3 will work well.

Side note for Sin/PT tanks. The absorb proc relic is certainly more viable (and I think BiS for PT's), though the Defense click will likely be better than the shield click. I sort of mess with my PT alt, but I'll admit I don't know much about Sin tanks.