Gotta say Superman. Just from reading the Morrison arc where he speaks to the world about standing up to the white martians made me realize that a team like the JLA needs Superman. Not just for his power, but for what he represents. I mean wouldn't you want the guy who stands for Truth, Justice and all that stuff to be part of the Justice League?
Now in terms of straight practicality the guy who needs to be on this team is Batman. He's the go to guy for whenever something strange is going down and all of his resources just can't be beat