Best Pattern for DAO

I would prefer to go with Enterprise data block from P&P as most of the plumbing code is done there and extend my apps further. 

But to answer your question , if you are building a DAO from scratch then you will find yourself moving towards Abstract Factory to take care of multiple providers and multiple return types.

You may also have a look at the Provider model which is a combination of multiple patterns and implement your own DAL using this....

http://msdn.microsoft.com/en-us/library/ms972319.aspx

Also, now Entity Framework (which again is based on a combination of basic patterns) is available which is a good option as well.  So, you see, there cannot be one answer :).