The best advice I can give you is to go see a doctor.. a dermatologist. I had the same problem I used all the shampoos the oils everything and nothing worked until I went and seen my doctor and he refered me to a dermatologist and from then on I haven't had dandruff. Good luck
If you've tried to treat your dandruff symptomatically and failed, it might be wise to identify the cause (there are many) and use the specific remedy for that cause. Your physician will be able to easily do this for you.
The best dandruff shampoo's are either selsom blue or tea tree oil. you can get selsom blue at any grocery/general store and tea tree oil is at sally's beauty supply. You can also ask them there if they have any other recommendations...they'll know what will work best.
Humans will always have dandruff no matter how much you try. If you want to mostly get rid of that visible dandruff and the dandruff that fall out of your hair when you scratch your head, selson blue works pretty nicely.