My friend is a graphic designer. He completed his studies in Graphic Designing from Toronto itself. According to him, Sheridan College in Oakville is the best choice for graphic designing courses, followed by George Brown College. Centennial has a dedicated school for this subject. 

Seneca and GBC offer graphic designing lectures on PC, while Centennial uses MAC for learning purposes. MAC is more popular among graphic designers because it offers a smoother experience for many design softwares. Even if such softwares have a PC version, they require less keystrokes/steps to complete a task on MAC.