Because Rooney and Ronaldo both have so much talent and are so young, I think they are the best canidates. Maybe Spector as well. Though Giggs is not having the best years of his life, for so long he was just a terror to opposing teams on that left wing. A good winger like that, who can create as well as exploit chances, take freekicks, pass, etc...is a rare commodity. That kind of talent gets you starts.
Doubt it'll be Ronaldo. I think he'll go back across the channel at some point but Rooney may stay around. On the other hand, most quality players these days want "a new challenge".