Devotion Aura

Typically when you make a "this is the X that Y should use" comment, you should be prepared to be downrated. All spells and abilities are situational, and a choice should be made every time which aura to use. Remember that you're not bound to any one aura: you can actually change them in fight. Actually, it's often pretty important to do so!

Let's round the benefits up here, from a tanking point of view:

Devotion Aura
This is still the prime tanking spell. Why? Because it typically reduces incoming damage, and that what a tank is for. A dead tank is a bad tank, and this aura actually helps preventing that from happening. According to sources like Elitist Jerks, personal experience and guild member's experience, Paladin tank threat is not an issue at this moment. If 4 mobs are beating on you with melee attacks and you don't have threat problems, why do that minimal amount of extra damage with Retribution Aura, when you can help the healer out with taking considerably less damage?
The biggest drawback of this aura is that it doesn't work against magical attacks, of course.

[spell=7294]
This aura right here. You listed the benefits of this aura in your post: doing some AoE threat. It only does a minimal amount of damage, but naturally the threat is boosted. Also, it's passive damage: you don't have to do anything to deal it except having the mobs beat on you. Retribution Paladins used to have a talent to increase this aura's usefulness, but they do not any more. So it's fair game to any Paladin. It's better for AoE situations which are easy. Again, if you're in a tough heroic or in a raid, why do a little more threat instead of mitigating more damage.

[spell=19746]
Not directly useful for tanks, but it's a group thing. If you, as a tank, having an easy time surviving but the casters are getting hit by whatever mobs or bosses are throwing at them, this is the right choice. Especially healers benefit from this a lot, because less cast time reduction means faster and more heals, which equals people staying alive easier.

Crusader Aura
If you have enabled this, you have forgotten to change to a useful aura (safe for situations in instances and raids where you have to mount up).

[spell=19891]
Basically the magical damage variant of Devotion Aura. It reduces magical damage not only to the tank, but to all party and raid members. When in a magic-heavy fight where survivability might be an issue, this is the prime choice. Even if the tank doesn't suffer as much magical damage, but the other group members might, this could be a good choice. However, keep in mind that if they're suffering a lot of pushbacks because of it, Concentration Aura might be better.

In short, not one aura is "the" aura for any specific type of Paladin. As a Retribution Paladin in a difficult instance, I would still use Devotion Aura to help the tank(s), or perhaps Resistance Aura in magical-heavy fights. You should decide upon the most useful aura when the situation changes. Don't use just one aura all the time,... on a phase change, when switching from physical to magical damage, change aura's. If you outgear the place, or there's no need for worry, feel free to use Retribution Aura for that extra damage.

It also grants 4075 armor to all raid/party members within 40 yards. And the icon has a shield with blue back ground. And the sarcasim is LOLing obviousness. But its not that obvious because its not the only aura you will be using depending on the fight.