What is the recommended metro ticket should i buy in Guangzhou airport if i will stay 8 days?

You are suggested to buy the ordinary transportation card of GZ and Shenzhen.
For HK, if you stay there for around 3 days, you can purchase the Airport Express Travel Pass. It contains one or two Airport Express single journey(s) plus 3 consecutive days of unlimited travel on MTR (except East Rail Line First Class, Lo Wu and Lok Ma Chau stations), Light Rail and MTR Bus (Northwest New Territories).
If your stopover in HK is within 3 days, just buy the Standard Octopus Card. For adult, the deposit is 50 HKD and the initial stored value is 100 HKD.