The essential oil from the leaves of lemon balm. It has a sweet, fresh, citrus-like fragrance that is lovely to smell but when it comes to skincare, it's a mixed bag. Thanks to its tannin and polyphenol components, it has antiviral, antibacterial and antifungal effects. In-vitro (made in the lab, not on people) studies have also shown that it has high concentrations of antioxidants.
The downside is that 50-70% of the oil consists of fragrant components (citronellal, geranial, and neral) that might irritate sensitive skin.