When the film comes out, star billing onscreen should be conveniently alphabetical:
The first three are obvious, because they topline their own films:
Renner next (he's relatively unknown, BTW)
"and Mark Ruffalo" next
Not sure where I would put Samuel L. Jackson's name. Perhaps before Ruffalo.
Clark Gregg (Agent Coulson) can go next among a list of other supporting players.
This, of course, covers only the cast members known today.