Some years ago I visited my in-laws in Hong Kong. Hong Kong is a small and densely-populated, but because its hills are steep and rocky, and the soil poor for farming, they were never disturbed much except for collecting firewood. There are still large expanses of mostly undeveloped land, about 60% of the colony's total. Hong Kong is a refuge for plants (and a few animals) as well as people.
The native habitats in Hong Kong are typically scrub on rocky hillsides, jungle, and low open woods. Frequent typhoons probably prevent tall forest from ever getting established.
China sits at a fairly low latitude, and mountains and deserts prevented the ice sheets from disturbing the flora during the Ice Ages. Although thousands of years of farming have wiped out most of the native vegetation on arable land, the remaining flora is richer in species than Europe or most of North America.
Some of China's most interesting plants are relicts (plants that have been reduced to small populations, often as a result of climate change). Several of Hong Kong's native Rhododendrons fall into this category. They survive on the hills, which, catching the cold front that spills out from the north in the winter, are noticeably cooler than the lowlands; on rare occassions they are coated with frost, despite sitting in the tropics and not being all that high.
Rhododendron hongkongensis is the rarest of the relict species; it has been found in only 6 locations in Hong Kong and neighboring Guangzhou province. It looks like and is closely-related to R. ovatum. It doesn't look much like the Rhododendrons most gardeners are familiar with; it's a member of the Azaleastrum Series. They're not Azaleas, but they are somewhat intermediate between Azaleas and other Rhododendrons, with fairly open, almost flat-faced flowers. Some of them including this one are mildly fragrant. I didn't see this one in Hong Kong (it would be hard to find) but I actually have a specimen courtesy of the Rhododendron Species Foundation. Although technically a tropical plant in the sense of being native to the tropics (Hong Kong is just south of the Tropic of Cancer), it is hardy in Seattle.
Another Rhododendron you might be lucky to spot in the undergrowth of the jungle growing on Victoria Peak is R. championiae. This one is a member of the Choniastrum Section, related to the Azaleastrums and mostly found in the Asian tropics. Many of them are fragrant. A few of them are surprisingly hardy to cold, but I don't know how hardy this one is.
Both of the preceding Rhododendrons look somewhat like Azaleas, but Hong Kong has a few real Azaleas, such as R. simsii. This species with medium-sized bright carmine blossoms is widespread and common in south and southwestern China to northern Burma and Thailand. It is believed to be one of the ancestors of the Belgian Indica Azaleas that are grown in hothouses, but some forms of R. simsii are hardy enough to grow in the Pacific Northwest as well as the southeastern USA. In Hong Kong you can see it on some of the hills.
An Azalea I saw in cultivation in Hong Kong has quite large purple flowers in early winter. It looks a lot like some of the Southern Indica Azaleas and is probably related. It must be very heat tolerant, whereas it was growing near sea level in the hot, humid city.
Native conifers are dying out, probably because of the heat. I saw scrubby pines growing on the hills and even a few Chamaecyparis here and there. More common in cultivation is Araucaria heterophylla, from Norfolk Island. This strange Gondwanic conifer is usually seen in the USA and Europe as a houseplant, but in south China it reaches its full size, when it assumes a fastigiate shape vaguely like that of a Lombardy Poplar. One common conifer that might be native (somewhere in south China, anyway) is Podocarpus chinensis, which looks like a shrubbier, smaller-leaved version of P. macrophyllus; they are probably just different forms of the same species. Growing in the hot, humid tropics it would seem as though it should be tender, but many gardeners in the southeastern USA claim it is actually slightly hardier than the already surprisingly hardy P. macrophyllus.
What most people mistake for conifers in Hong Kong are more likely to be species of Casaurina, strange Equisetum-like trees from Australia. They are not conifers, and they are not closely related to anything else. Eucalyptus is another common import from Australia.
Schefflera octophylla is common. It occurs in cultivation but is more common in the understory of the native low jungle. It's a small tree in size, with modest-sized leaves, and otherwise looks like a typical Schefflera. I don't know how hardy it is, but some of southwestern China's Scheffleras are hardy enough for temperate maritime climates such as in the Pacific Northwest of North America and coastal western Europe.
Another common plant with Gondwanic affinities is Dianella ensifolia. It is not as large or pretty as D. tasmanica, lacking the intensity of color of flower and berries, but it lives in a much hotter climate, and might be suitable for breeding heat-tolerant architectural plants for places like the southeastern USA. It grows in open woods and under scrub.
In the winter, the hills of Hong Kong Island are cheerful with the white blossoms of a Camellia-like plant. I think it's actually a species of Gordonia, although Camellias do occur in Hong Kong. The only Camellias I saw were at the Hong Kong botanical gardens, and they were in poor shape.
Gardenia jasminoides occurs wild in Hong Kong. Compared to domesticated plants, these tend to have large, single flowers. The produce a fruit (probably not palatable) that is used in Chinese herbal medicine.
Hong Kong does not have many native palms, but the Rhapis palms are probably native (if not to Hong Kong then probably nearby), as well as perhaps Caryota ochlandra which is common in cultivation. Other species of Caryotas are rare but I did see a few. Coconut palms (Cocus nucifera) are not particularly common but I did see a few stunted specimens; Hong Kong actually gets too cold for them in the winter but because the lowlands are essentially frostless they survive even if they don't thrive. Trachycarpus palms which are common elsewhere in China don't seem to occur in Hong Kong at all; it is probably too hot for them for most of the year.
Hong Kong also has vast numbers of non-native plants which have escaped cultivation. They are too many to name, most of them are tropical and quite tender, and they tend to be the same plants that are now pan-tropical.
One of the most conspicuous is Bauhinia x blakeana, with enormous blossoms whose fragrance mixes with the street smells to give much of Hong Kong a characteristic odor of perfumey, exotic incense mixed with sewage. The blossoms of B. x blakeana are the floral emblem of Hong Kong, and an image of one occurs on the local flag. It is called the "Hong Kong Orchid Tree", and perhaps was bred there by crossing two different species, but I don't think either of its parents is a native of Hong Kong. It seems to be quite sterile, as the trees never seem to have any pods, but one of its parents, with smaller, less showy blossoms, does go to seed.
Bo trees, Ficus religiosa, are quite common and reach surprisingly large sizes for a climate with typhoons. They were probably brought by Buddhists, for this is the tree that the Buddha was sitting under when he achieved Satori; the name "Bo" is derived from "Boda", which is usually transliterated as "Buddha" (I think "Boda", with a palatalized "d", is closer).
While Hong Kong is surprisingly well landscaped, there isn't much variety of native or exotic plants in most of its parks. The gardens in the old British colonial parks are an exception; that's where you can see a great many exotic species and they are worth visiting. Although small and now surrounded by millions of people they are in good condition.