+1 relic weakest upgrade for tanks?

[Maelwys]

The shield is an extremely potent upgrade. I agree the ring is also, but I wouldn't go past my first 3 upgrades without getting +1 sword/shield and heros ring (unless you get one of those three as a drop).

As mentioned above, the Shield upgrade gives you +1.2264% Block Strength.

If we assume your block rate is about 24-25%, this equates to a boost in damage mitigation of around 0.3% over time versus non-magic frontal hits.

If you consider that to be worth your first week's myth, then by all means go for it.
Personally, I'd rather go for the Ring --> Body option for my first 4 weeks.

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Shield is only NOT BiS if you ahve access to turn 5 loot.

Regarding BIS:

Post Turn 5, the Allagan Shield is undeniably BIS.

Pre-turn 5, if we ignore that using the Holy +1 shield over the Allagan requires you to use food for Turn 5 (you'll likely be eating Toast anyway for the +Vit and Parry) then it will grant you an extra 7 SS, which is enough to bring your GCD down from 2.162 seconds to 2.156 seconds.

If your reflexes are high enough to notice a 0.006 second GCD reduction, then more power to you. Personally I very much doubt that the server clock can even register a 6 millisecond time interval, so would consider spending a week's worth of Myth on that to be largely pointless (assuming that you already have the Allagan Shield). YMMV.