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.
Hibiscus sabdariffa and Hibiscus rosa sinensis seems to have not only potent antioxidant qualities but also wound-healing ability and, when protected from exposure to light and air until used, this type of hibiscus helps interrupt melanin synthesis, leading to improvements in sun-damaged skin (Sources: Pharmaceutical Biology, August 2013, pages 941-947; and Indian Journal of Pharmaceutical Sciences, January 2013, pages 45-49).
Hibiscus abelmoschus seed extract is a potent antioxidant that has in vitro research (that was supposedly duplicated in vivo, meaning on skin) that it can control the release of growth factors responsible for fibroblast production, fibroblasts being cells that make collagen. Although that's intriguing, this research comes from a raw material supplier, not an independent third party (Sources: International Journal of Molecular Science, January 2012, pages 628-650; and International Journal of Cosmetic Science, December 2009, pages 419-426).
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 irritation, hibiscus isn't among the top antioxidants we'd recommend you look for when shopping for skin care, though the fragrance concern is primarily when the whole plant is used rather than isolated extracts, which may be less (or minimally) fragrant.