A few years ago when I was studying French, I escaped from the modern languages department for a while to take a module in theology. The course followed the history of the Church of England from the Reformation. When asked by a student, not me, what, if any, were the unique contributions of the C of E to the Christian tradition the lecturer gave has his opinion that the Anglican Communion was the only "reformed" church which had avoided the splits to which Methodists , Baptists, Brethren and others have been prone.
Rowan Williams has struggled heroically to hold our Communion together against those who feel that their views on homosexuals and women are the only possible ones to hold. David Steel once said in parliament that he wished he was as certain of just one thing as Mrs Thatcher was on everything, and I'm sure Williams has felt much the same about those who wish to dictate the direction of the Communion solely according to their own prejudices.
In addition to his efforts to preserve the liberalism of the Communion Williams has, in my view, ranked alongside other great archbishops such as Temple and Runcie, (and Bell who didn't become archbishop because he spoke out against the saturation bombing of civilian populations,) in openly opposing the Iraq war, and speaking up for the poor against the unfairness of and damage done by the present government's cuts and changes to welfare provisions.
I'm not sure whether it is theologically possible to jump from a mere "Reverend" to a "Most Reverend" in one leap, but if it is, then Giles Fraser seems to me to be the most obviously qualified replacement.