Logically you say Superman. I wouldn't do too much in the movie that signposts everyone else, just the odd throwaway line about Gotham City or see someone use a Waynetech laptop, that kind of thing.
They'll reboot Batman soon enough after the Nolan trilogy. A little cameo of Henry Cavill's Clark Kent asking Bruce Wayne a question at a press conference would do it. You dont even need to use his name. They'd have a fun tete-a-tete. "Mr Wayne, you seem to have a pretty hectic lifestyle. Running Waynetech and all your night-time antics, how do you do it?" "What can I say? I'll sleep when I'm dead. The idea of free time is positively alien to me at this point!"
Give me Wonder Woman taking on Hydras and Giants and Aquaman in an underwater Game of Thrones senario, the Flash doing Doctor-Whoesque time travel stuff and the GL franchise being rehabilitated into being watchable, then we'll talk JLA. (Which would be White Martians invading. Superman and Batman get beaten up, round up some reinforcements, EVERYONE gets beaten up, but the White Martian's Green martian prisoner, J'onn J'onzz gives them the edge they need to break out and win).
Then I want a JSA movie that shows a bunch of old men coming out of retirement to kick evil's ass one last time...(by the end they'll decide to train their progeny, of course...)