I think of the current lot Shoaib Malik would be a good contendar or even Shoiab Akhtar. Shoaib has improved a lot since the world cup, he looks commited and has taken the responsility of the sernior bowler well. His attitude has improved as well. He is an attacking bowler and it might just be the aggession needed to lead our side.
Yunis would be the best bet if Inzi retired. He seems like a really well grounded intelligent guy. Unfortunately he is not a good enough batsman to play on the team regulerly. So I would say Shoiab Malik or perhaps Taufiq if he sticks that long.
By the way people laugh at Zia and I agree he is'nt a quality bowler yet. But apparently he is a really hard working and honest guy so you never know about his future. But his dad would have to quit the board first.