As much as Buffy inspired Angel to become a better person in the first place, and as much as Doyle put him on the path that would, eventually, someday, hopefully, lead to some degree of redemption and peace of mind for him, without Cordelia Chase, Angel would be a jibbering idiot sitting in a dark corner drawing dirty pictures, while a one armed Wesley and Gunn run around Los Angeles trying to keep the demon population under control. Or am I the only one here who remembers Season 3 "Birthday?"
So, Cordy was, undoubtedly, the most important person to Angel. Not only did she save his sanity, she saved him having to kiss Doyle in order to get the gift of prophecy