It appears that many of the critics' reviews out there are casually giving away the basic plot of the film. The real joy, though, lies in slowly unraveling the veneers of this deliciously layered narrative yourself and anyone depriving you of that pleasure is committing nothing short of a sin.
Keeping that aside, the beauty of Parasite is that it takes on the garb of many genres at once. On the face of it, it appears to be a black comedy. As the story progresses, you slowly start moving towards the edge of your seat as it shifts gears into thriller mode with an impending sense of danger looming large. Then, it acts as a smart socio-political commentary on the economic divide and class rage between the rich and the poor. There are also elements of horror running through its veins, though not the "paranormal" kind but just due to the misactions of its painfully "normal" people. Ultimately, how I see it, the film is crafted with the sole intention to shock the hell out of you and blow your mind out of proportion. And boy does it succeed so well! In fact, the writing in this blazingly original piece of work is so sharp that there are moments where you judge yourself on moral grounds as you giggle at the exceedingly vicious actions of its twisted characters.
I'm going with 4.5/5 for Parasite. It feeds on your imagination, fear and insecurities unlike anything you've seen of late. This is a parasite you wouldn't mind hosting.