If we had to pick one universal language, I'd say English.
It's already one of, if not THE most widely spoken language in the world.
America is already a very influential country in a lot of the world.
I can't speak for how easy it is to learn, since I've been speaking it my entire life.
Or maybe, maybe, Greek.
I dunno, I just really like the way it sounds.
...maybe that's just 'cause I've been stuck with it for the past two years.