When trying to establish a versatile and powerful illusionist, who reigns supreme? The Illusionist Wizard? The Bard? A Sorcerer of some description? I've googled around and haven't found much of a consensus.
Illusionist wizard. The bard class has a good selection of illusion spells, but bards have a much smaller number of spells known than wizards, so if they want a robust selection illusion spells they come at the expense of general versatility, something wizards don’t have to worry about as much.
The bigger thing, though, is the illusionist subclass features, which boost illusion spells (obviously). I don’t think any other subclass has the same focus.