To me he drew definitive versions of all JLA characters. Dillin (JLA), Neal Adams (whole DCU), Jim Aparo (Batman and others, especially in B&B), and Curt Swan (Superman) were the ones that created the looks we got used to being the definitive versions of characters as we knew them in the 70s and early 80s. I mean before George Perez came along.
Of course there were other definitive artists that are so noted by their inclusion in style guides, such as Jose-Luis Garcia Lopez, but they were aping the looks that Adams and Dillin created.
Dillin's Green Arrow was perfect but it was also a lift of Neal Adams' version. Back then the goatee and classic costume worked in a way they never have since. All thanks to Adams and Dillin.