What is the best thing for dark under eye circles? best product to use?

Many products, but: I don't know any that work well. Some people inherit dark circles to some degree, but other times it's from anemia or environmental factors such as lack of sleep or allergies of the eye/nasal sinus area. Try getting better sleep and addressing whatever allergies you may have, both in your eyes and in your sinuses, and get a medical checkup if you haven't had one recently.

Dark circles- Eyes : Under eye circles have multiple causes including increased pigmentation, sun exposure, dilated blood vessels, blood pooling from allergy flare, drinking alcohol. Genetic and hereditary factors also contribute. Correction depends on cause(es). A few topical creams can be helpful. Use of fillers like belotero, juvederm, Radiesse (dermal fillers) are helpful as are peels and laser resurfacing.