If this question was asked 7-8 weeks ago I would have said Scully.
Now, there are three players... Deledio, Cousins, Cotchin.
Most people would pick Cotchin over Deledio right now, but I think that's partly because people prefer things that are new over the old, if they're on similar footing of course.
Cousins is one player I'd do anything to keep in 2010 and 2011 because he could have a massive, massive influence on the future direction of the culture at our club. This is debatable, I know, but I rate his qualities and potential impact for the future very highly.
Cotchin could bring 10 years of setting a new standard, so I'd pick him above Cousins. If you wanted one player for 2010 without looking to the future as much (and looking mainly on-field) I would pick Deledio 1st.
Depends on how you interpret the question as to what your answer should be of those 3 IMO.