if you are coming from futian, lowu or sha tau kok, a43 is the best, sheung shui mtr station has a bus stop for a43 ( a3 exit closest , opposite landmark north and ) generally 40-45mins to airport, frequency is at 20 minutes, rarely hinder by traffic ( as it goes via tai lam tunnel which is underused probably due to relatively high price for cars at HK$36 )