Full Idea
Some metaphysicians take the highest division to be between abstract and concrete entities, others take it to be between universals and particulars (my own preference, though it is not crucial), and others between necessary and contingent entities.
Gist of Idea
The top division of categories is either abstract/concrete, or universal/particular, or necessary/contingent
Source
E.J. Lowe (The Possibility of Metaphysics [1998], 8.3)
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.180
A Reaction
The first division may be blurred, and I am doubtful about universals, so I favour the third. Intuition tells me that there is nothing more basic than the distinction between what is true in all worlds and what is only true in some. The former is bedrock.