I think the truly greatest illusion is the illusion of time, because so many people assume it is absolute and the present is the soul seat of reality which is a complete fallacy. Time exhibits no energy or mass it is just an illusion that some entity is flowing like water down a river. Time is no more than a static dimension like the other three physical dimensions we are accustomed to in the real world.
I can recall a British sitcom, One Foot in the Grave when Victor was speaking of this in bed with his wife Margaret, and he said
"When you think about it, nothing ever exists, in fact. I was working this out in the post office as I was waiting for that woman to finish twanging her elastic bands. The future doesn't exist because it hasn't happened yet; the past doesn't exist because it's already over. But the present doesn't exist, because as soon as you start to think about it it's already in the past, which doesn’t exist any more."
I was quite happy to find these few lines of the script on the internet. But I think he has missed the point a bit because every exists, even events in the past have not diminished their existence because we deem them to be in the past. To me it would be a akin to travelling from London to some mysterious destination by train and while you are half way you assume London no longer exists because you have left it and that mysterious destination doesn't exist because you have not reached it yet, in which turns out to be Birmingham, but you were not to know that until your arrival. Kind of like someone living in the 1920’s believes the Great War doesn’t exist because it is history and any future impending war does not exist because it is in their future, which eventually pans out to be World War 2. But just because they have not reached it yet doesn’t diminish its reality jus as not knowing know you were heading to Birmingham does not diminish the reality of that place.