I would say J'onn, just because he pretty much is the JLA, the heart and soul and brains of it all in one. It's as constant a thing as you get, pretty much anywhere in comics. In general I like to see the team as the Big 7, MM along with Supes, Bats, Wondie, AQ, GL, and the Flash, and then Plastic Man's always there because any chance you have to put Plastic Man in a comic book, you should take. I did like the Morrison/Kelly-era portrayals of the team, with that group as sort of the core members of a continually rotating and updating roster of members changing as circumstances dictated. At any given moment you'd have Orion, Barda, Steel, Huntress, Green Arrow, Hawkgirl, Firestorm, Zatanna, any of an entire list of great characters involved.