people need to stop being so butthurt about other peoples opinions. just because you use (or do not use) a certain skill, doesn't mean everyone else does. everyone has their own way of playing skyrim differently.
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!