This is a tough question in a way because there are many different theory's for this question.

I read an article in BTM that I thought was a good way to drill up an arsenal.  It's a 6 ball arsenal.  There are two sets of three balls.

SET 1 heavy, med and light oil balls with a power stacked layout(same layout)

SET 2 heavy, med and light oil balls all with a label layout(same layout)

The different type of ball will show the difference.

To me this makes the most sense.  This 6 balls arsenal gives you 3 balls that are very powerful for the certain amount of oil and the other 3 balls give you control.

This to me is a simple approach that I plan on doing.  The only thing that I would probably change is the dry lane ball.  How many real dry lane balls do you need to be powerful on the back end?  Who knows.