Considering native Hawaiian flowers - I love the smell of hinahina ku kahakai (Heliotropium anomalum), and na'u (Gardenia brighamii). Both of these species are native to Hawai'i. There are many gardenia with similar fragrancesbut Na'u can only be found in Hawai'i, which to me enhances its fragrance. Gardenias all have that simple creamy-white-flower smell, but there is something about them that seperates them from flowers that have similar fragrances (e.g. puakenikeni, arabian jasmin). Hinahina ku kahakai has a very faint but unique scent somewhat reminiscent to Osmanthus fragrans.

If I counted non-natives, among my favorites would be; Osmanthus fragrans and roses. Roses have that classic fragrance that no other flower (that I know of) can get close to. To me Osmanthus fragrans (I have heard of it being called "kwai fa") has to be one of the most amazing smelling things on Earth.

I think my all-time favorite is white ginger. It's sweet and musky, but not TOO sweet or perfumey.