I hope that I'm not overstepping myself, but I'm not sure that the question is entirely answerable in this format. It's difficult to sum up one individual's importance to the whole of European history in a few paragraphs (or a few hundred pages), and it would seem to invite speculation and generalization to try. Generally speaking, historians aren't terribly interested in deciding how important someone is (politically? militarily? economically? culturally?). If some other user thinks otherwise, I'll be glad to stand corrected. Or if you're just wanting a summary of his life and career, that could be managed rather more easily.

However, if you're after reading on the subject, I can wholeheartedly recommend David Crouch's William Marshal: Knighthood, War, and Chivalry, 1147-1219. Crouch is pretty much the godfather of 12th century aristocratic studies, and the book is superb.