What's the best transport mode for me to travel to Shantou from Hong Kong now ?

Sorry to tell you ne bullet train between Shenzhen and ST.
Well, you can take a ferry from HK-Macau Ferry Terminal to Shekou Port, which is available from 9:00 to 19:30.
After getting there, you can take a taxi to Shekou Bus Station, from where you can take a bus to ST. The bus is available from 6:10 to 19:20 and takes CNY120.