well... for specific unis, it's different. but generally, medicine, dentistry, global business courses and law courses are the most competitive ones.
stats:
HKU admitted 4 of the 33 5A* achievers in GCE A-levels last year
dentistry for HKU: dont ever think about it without 3A* if u are at the last round interview
global business: median of last year's admission for HKALE was 5A in 3AL, 2AS
[unofficial estimation by unis: GCE A* is roughly equal to HKALE C]
other courses are also competitive localised(i.e. not many international applicants) like journalism.
sorry for telling stats about medicine. i'm a med app so i have to stick to these figures :P
econ and finance are also competitive. it varies from uni to uni. again unofficial rankings of business-related courses by hk students:
HKUST Global Business > CUHK Global Business > HKU International Business Global Management (IBGM) > HKU BBA(Law) > HKU BBA(econ and finance) > CUHK BBA(account and finance) and so on.
HKUST would be the most popular business course in hk since it's "renowned" for its heavy homework load. (some say UST = University of Stress and Tension :P)
HKU is prestigious because of its name and fame and internationalism. also their hall culture, social network is much broader.
CUHK is well... not the 1st or 2nd but also not bad
EDIT: i forgot to congrads for you LSE offer! it's very nice to meet very capable ppl like u with LSE offers! my god getting an offer from a uni is like a dream...
lol u should study med in CUHK!! it's a pity that u gave up the offer!