Full Idea
A strong case can be made for the claim that it is the primary subject (to hupokeimenon) that is substance (ousia) to the fullest extent.
Gist of Idea
The primary subject seems to be substance, to the fullest extent
Source
Aristotle (Metaphysics [c.324 BCE], 1029a01)
Book Reference
'Philosophy: a Guide Through the Subject', ed/tr. Grayling,A.C. [OUP 1995], p.174
A Reaction
It seems to me that 'ousia' is best translated as 'being'. Aristotle eventually rejects the view in this idea, which is roughly the idea of that being is mainly the bare substratum.