i voted for illusion. it WOULD be a highly useful skill, IF the spells didn't have a level cap. for example, a calm spell = 'creatures and people up to level 9 wont fight for 30 seconds', and the opposite with a fury spell. great, but i am level 25? and so are the majority of my enemies? illusion spells always seem too weak to be used when i want to use them. 'enemy resisted calm' pretty much all the time. also, harmony, the master level calm spell, requires 100 in illusion to be used.. but only works on people/creatures up to level 25? and i'm pretty sure that by the time people get a skill to 100, they have almost definitely gone past level 25. illusion would be amazing if it didnt have a level cap, but it does, so i hardly use it seeing as the majority of my enemies are too strong for it to work! if you really need to calm something down, voice of the emperor or command animal works much better than an illusion spell! only useful thing about illusion = muffle, and the quiet casting perk!
I have to correct you here on your misguided take on Illusion. You can take the basic Calm spell, use the Kindred Mage perk (40 skill, and you take Animage on the way), use the Hypnotic Gaze perk (30 skill), and use the Dual Casting perk (20 skill). Calm goes up to level 9 on its own. Add in Kindred (+10 levels) and Hypnotic (+8 levels), and you can hit level 27 humans, and level 25 animals.
Oh sure, you say that isn't good enough? Well how about a Dual Casted Calm spell (mind you, this only takes 40 Illusion skill and 4 open perks, so by your level 5 you can be doing this), 2.2 times the spell magnitude, meaning you can now hit level 59 humans and level 55 animals! So what were you saying about level 25 and the spells not working? At that point your only problems are things unaffected until you get the 90 skill perk Master of the Mind.
If you want to use the higher level spells, stick with Pacify, because Harmony can't be dual cast, and if it can, no one has enough Magicka for it anyway. Pacify can hit level 38 humans and 36 animals with Kindred and Hypnotic. Dual cast, this brings you to 83 for humans and 79 for animals.
For perspective: Frost Trolls are level 22, and they are the highest level animal (aside from passive Mammoth level 38 which you can dual cast calm), so you can hit that with your basic calm. The highest level Bandit is a Chief at 28, 1 level over your basic calm spell single cast. An Arch Necromancer caps at level 46, so you can easily hit these with a Dual Cast Calm spell as well.
Not only that, but as you said Illusion also includes Muffle and Quiet Casting, as well as Invisibility which is also quite nice.
TL;DR You can take 4 perks and use the Apprentice Calm to hit every enemy you will run into effectively, humans and creatures, that Calm is supposed to effect.
EDIT: apparently I was ninja'd while typing this out.
