Agree with Scotland, IC GS will be my first choice and the 2nd choice will be CP Causeway Bay, but it depends on what you want to do during daytime. If for shopping, then you should choose CP as Time Square shopping mall is next block and there are many department stores and high-end boutiques nearby. If this will be the first time in HK, then you should choose IC GS as you might want to do a bit of sightseeing. Walk around the waterfront at night from IC GS all the way to IC HK and even further to Star Ferry Terminal and appreciate the amazing view at night.

For the transportation from CP to Airport in the early morning, it is better to take a taxi and there will be no traffic jam at 5-6am on weekday (rush hour start from 8am). The taxi fare will be a bit cheaper if you ask the taxi driver to use the Cross-Harbor Tunnel at Causeway instead of the expensive Western Harbor Tunnel.