Wildstorm will always be the house that Ellis built, to me. The direction the line took was so heavily influenced by him; remember that he was there from early on, with DV8, but Stormwatch evolving into Authority and Planetary...that really defined the whole approach. It went from hodgepodge of Marvel and DC homages with a 90s scifi flare to a paranoaic social scifi experiment.
His transformation then allowed a bunch of brilliant creators like Brubaker, Ennis and the Chief to take it forward.
He's done most of his best work there, I think (Stormwatch, Planetary, Desolation Jones), so he both defined it and it defined him.