Which is the good octopus card for tourist?!

If you will visit most of the places by MTR, it is suggested to buy an Airport Express Travel Pass, so that you can save a lot during the first three days when you take MTR.
If you will visit the attractions by buses, the standard Octopus card. In a word, it depends on where do you want to go in HK.
All these cards can be bought at the Airport Station of MTR and you can get your remaining money back at that station, too.