Answer: Cameron
"The 1961 Ferrari 250GT California. Less than a hundred were made. My father spent three years restoring this car. It is his love. It is his passion." - Cameron
"It is his fault he didn't lock the garage." - Ferris
The movie should have been titled "Cameron's Day Off" because, at its nub, this was what the movie was all about: Ferris trying to get his friend Cameron to face his fears so he can rid himself of all his psychological issues wrapped up as chronic hypochondria. (Cameron believes his Dad loves the Ferrari more than he loves Cameron - The Ferrari is a symbol of both Cameron's devastating misplaced attention and his son's sum of all fears.).
"Cameron, this is my ninth sick day. If I get caught, I don't graduate. I'm not doing it for me, I'm doing it for you." - Ferris
Not that Ferris was so altruistic that he (and Sloane) are not going to enjoy the day as well. Ferris' plan, of taking the Ferrari on a spin (and returning it unnoticed by Cameron's father) would go a long way to helping Cameron. The plan worked except for the parking garage attendants clocked up 170 miles on the odometer while our friends were enjoying the city.
(Incidentally, there were four Ferraris. The genuine one is on display in Cameron's garage and three cheaper copies - fibreglass replicas built on Mustang mechanicals. One was used for the friends' driving scenes, one with beefed-up suspension for the parking attendants' escapade and one that crashed).
There was a moment in the art gallery when Cameron examines Georges Seurat's "Sunday Afternoon on the Island of La Grande Jatte". Cameron sees himself in the painting, as the little girl dressed in white, who is holding her mother's hand. The mother isn't paying any attention to the girl and she seems upset. As the camera pans in on the girl, she is reduced to a group of dots, demonstrating how Cameron identifies with her discomfort but also with her absence. His sense of self has been destroyed by all of his anxiety. This painting signifies Cameron's lack of identity as he has been reduced to a collection of dots. It is at this point we realise that Ferris' monologues about his friend are accurate.
Back at the garage, Ferris and Cameron jack up the car and run the car in reverse, hoping to take off the extra miles. It doesn't work. At this point, Cameron starts to get angry. "Who do you love?", Cameron rhetorically asks his absent father while kicking the front bumper, damaging the car.
"Who do you love? You love a car!"
Cameron does not hate the car, it's what the car represents: an absent pre-occupied father who loves an inanimate object more than his own son, and the fear and anxiety that such knowledge has shaped him. By kicking the car repeatedly Cameron, kicks it off the jack. It reverses through the back glass wall of the garage and crashes into the woods below. He sends his fears with it, and he realises he now has the strength to finally confront his missing, withholding parent.
This is not the way Ferris wanted Cameron to face his fears but it works nonetheless. There is nothing left for Ferris to do but to get home before his parents, to ensure his day off is not discovered. (We never find out what happens to Cameron, but we assume he has conquered all his psychological issues).
Cut to the final scene where Ferris is lying in bed, his parents happy that he is 'recovering' from his illness:
"Life moves pretty fast. If you don't stop and look around once in a while, you could miss it" - Ferris.