SPF (sunscreen) is the most important skincare product. It prevents UV radiation from coming in contact with your skin, and could prevent skin cancer.
Cleansing is also important, since you need to get all that dirt and grime off your skin. Although, some cleansers can be overly stripping and deprives the skin of your natural oils.
Moisturizer is important because it keeps your skin moisturized (obviously). Even if you have oily skin, you should still apply moisturizer. If you dry out your skin, you will just cause extra sebum production, which can cause more acne.