We arrive in HK airport at 6pm. How is the best / fastest way to get to the Canton Fair area.

Upon arrival, you can take long-distance bus and get off at Guangzhou Grandview Mall first. The bus schedules are from 07:00 to 20:30 with the frequency of around 30 mins. The duration is about 4 hours. It costs 250 HKD per person.
Then take subway line 3 to Kecun and line 8 to Xingang East Station, Exit A. After that, walk east along East Xingang Road around 450 yards (410 meters).