It's a Yellow-browed Warbler. The larger wingbar (on the greater coverts) is much too broad for an Arctic Warbler. You can also see that the bases of the secondaries are dark, creating a 'shadow' behind this wingbar - this is a very useful feature to identify Yellow-browed Warbler. Two-barred Warbler is another similar species, but should have a slightly narrower wingbar without the darker 'shadow'.
Also, the time of year is useful - Yellow-browed Warbler is a very common winter visitor to Hong Kong, but Arctic Warbler is a migrant and is extremely rare in winter (two birds this winter are the first January records ever I think).
