These are impossible questions to answer. The only viable answer is "it depends."
The types of questions you should be asking yourself are:
What are my storage requirements? - How many tables will there be? - How many records in each table? - Do I need to be fully ACID compliant? - Do I need full transaction support? - Do I care about foreign key constraint support?
What are the requirements of my business? - Am I willing to pay for a commercial database? - Are there any constrains on using open source software in my business? - What is my budgetary requirements in choosing a database?
What options are available from my hosting service?
If you have already chosen a hosting service for your application, and that hosting service offers MySQL only, for example, then the answer to your original question has already been chosen for you. The best database adaptor in that scenario would obviously be the MySQL adaptor.
I think you get my point. It matters very little about the adaptor, unless it's completely broken. What matters is choosing the database that fit your particular needs.
In most cases I choose to use SQLite for my development and test databases. This reduces the time spend setting up and configuring my database during the development stage to zero. I then normally choose MySQL for my production database. At the place I work they use Oracle for nearly everything so there is no choice to be made in that situation. The production database must be Oracle due to purely business constraints. Now it's pretty obvious that I wouldn't choose Oracle for my own personal projects due to the cost of Oracle and it would be overkill in most situations anyway.
What I think is really important is to make every effort to design your Rails application to be database agnostic. That way you are completely free to choose whatever database makes the most sense for your deployment needs.