which bus is best to go from hk airport to hung hom to catch 7:25am train to guangzhou?

Airport shuttle bus no. A21 can directly take you from the airport to Hung Hom Station. However, the first bus leaves at 06:00 from HKIA and the travel duration is around 75min. To catch your train at 07:25, time shall be too tight, unless the bus runs faster on the way and you have booked the train ticket in advance.

If you don't wanna take a risk in missing the train, you are advised to take Airport Express to HK Station within 25min and then transfer to Tsuen Wan at Central Station to Tsim Sha Tsui and then West Rail line to Hung Hom Station to catch your train. The MTR duration is around 50min!