It varies! If you have dry skin, moisturizer is usually the product you'll want to invest the most in. If you're acne-prone, soothing products and spot treatments will probably be your most important buys, etc. Having a system that works for you is really the most important thing, though—regardless of price. We love the Somme Institutesystem, but we also have nothing but great things to say about CeraVe Moisturizer and Cetaphil Cleanser.
If you have a specific skin concern, visiting a dermatologist is the best thing you can do, though. Products make all kinds of claims, but a doctor can evaluate what the roots of your skin's issues actually are.