Any of numerous types of fragrant flowers that convey antioxidant ability. According to Natural Medicine's Comprehensive Database, "The applicable part of hibiscus is the flower, calyx or sepal, leaves, stems, and seeds. Sepal extracts contain significant amounts of vitamin C, anthocyanins, and polyphenols. The sepal also contains the highest concentration of water-soluble antioxidants."
They go on to state that the seed and leaf of hibiscus also contain significant amounts of vitamin E and that hibiscus seeds contain the highest concentration of lipid soluble antioxidants.
Despite the impressive research building for various types of hibiscus, it remains a fragrant plant, with some types being notably more fragrant than others. Because fragrance is a source of sensitivity, hibiscus isn't among the top antioxidants we'd recommend you look for when shopping for skincare, though the fragrance concern is primarily when the whole plant is used rather than isolated extracts, which may be less (or minimally) fragrant.