Questions regarding PLD skills...

1) Shield bash vs Low blow. Should I utilize both or use one over the other? What are the pros and cons to both?

2) Silence. What's the point of this skill if I can just use Shield bash/low blow to counter an enemy skill? I must be missing something here.

3) what is the recommended defensive cooldown rotation? I usually start with rampart because it lasts longer which allows me to focus on dps. Once that's done I pop Sentinel then Bulwark when that runs out and finally anticipation. I throw convalecence when I'm around 50%hp to help out the healers and awareness every chance I get.

Any advice would be greatly appreciated!

The advantage to shield bash is it has no cooldown. I don't tend to use it often, but you could for example cycle through every add in a pull and stun them one at a time. I would probably go with low blow if you're just trying to say, quickly stun a trash mob to interrupt an aoe. Spamming shield bash on different enemies probably won't really do much good considering the lost dps as you cycle through them.

I don't think I've ever actually used interject to silence. I have it on a bar but not hot keyed and never really use it, maybe there are situations where I should.

As far as defensive rotations go, for me it kind of depends on the fight. I generally consider how long is the fight. For exmaple, if I use hallowed ground on the first buster, I may get a second HG out of the fight, but if I wait, I may not get to use it a second time before the cd is up. Same to a lesser degree with sentinel which has a shorter, but still substantial cooldown. I just try to juggle things so that I always have something up for every trash pull or tankbuster, and tank busters vary a lot by boss in terms of their frequency.

1) Shield Bash is a GCD skill while Low Blow is not.

This means that Low Blow can be used at any time and does not interrupt your combos.

However, Shield Bash doesn't have a cooldown so you can use it more frequently, such as to stun multiple targets during some trash pulls.

Typically, Low Blow is the superior option because it's more easily used. That, given with the infrequent need to stun targets, mitigates some of Shield Bash's benefits. Though, it's useful to keep both in mind depending on what content you're running and if some of Shield Bash's pros outweigh Low Blow.

2) Silence does not have diminishing returns. Meaning you can keep silencing targets repeatedly and they will never become immune. Meanwhile, stuns are affected by DR so their durations are reduced by 50% twice and then the target becomes immune until 1 minute after the last stun they were affected by. During that time, a stun will do nothing. As a result, this is why Silences tend to be quite short duration, they're pretty much just used as interrupts for players than actual silence effects.

Wanted to add, silence works against magic only. There are fights where it is NEEDED, most recent being Ultros that casts a 300% of its health shield (or something ridiculous like that) and Seiryu adds that cast AoE stoneskin that greatly hampers the DPS check at lowish gear/skill levels.

Low Blow > Shield Bash.

Low Blow is oGCD (as mentioned by Kalise), free, and has a short CD.
Shield Bash is on the GCD, breaks combos and costs a whooping 120 TP. (activates faster but you have to wait for the GCD to even use it.)

For your third question, there is no "best" defensive CD rotation. You tailor your usage to:

a - Nature of the fight. Damage intake over the fight's length.

Is it a pack of trash mobs? These usually have higher damage in take that gets lowered as mobs die off. So bigger CDs first and weaker ones later. I tend to pop Hallowed in one of those.

Is it a boss? The next point is more relevant:

b - Solo tanking vs co-tanking and what their job is.

If solo tanking, bosses that are designed to be with one tank tend to have relatively low damage or highly spaced tank busters. In the first, you want to use shorter CDs like Rampart first so they come back first. In the latter you want to mitigate more via Sentinel.

In co-tanking situation, it helps to know your tank jobs' capabilities.

PLD has very effective buffs but very long cooldowns. Warior can cheese through a lot with its cheat death holmgang (which has the same CD as your Sentinel) while DRK will rotate its relatively shorter CDs more with TBN on top.
You probably want them to start the fights so whatever they use comeback earlier. You can also Cover them to take a buster in their stead without tank swapping, though this option is not always recommended because less Shirks mean less aggro boosts and cover has other uses that can help non tanks handle mechanics where you take the damage instead of them and at a much lower healing requirement. So it is fight dependent.

EDIT: PLD has the unique ability of CD sharing through intervention, in many situations where both tanks tank damage or share a buster to split its damage it might be worth considering a CD share to save a CD on your partner. My static does this in O12s as a means to save a CD for my co-tank to use to mitigate more damage elsewhere.

Hope this helps

Low blow is preferred since its OGCD.

I only use shield bash when I have to stun 2 or more enemies.

Interjection is very useful for enemies casting spells on you.

Defensive cooldowns vary depending on the fight/content and is something you really pick up yourself as you learn the fights.

Thanks for the advice everyone!