MLS, but soccer is a truly global sport and includes much, much more, including the world’s best leagues (which MLS is not a part of) and national teams and international tournaments.
This is hard for traditional American sports fans to wrap their heads around, because they’re used to the local US league being the be-all, end-all of the professional end of the sport. Soccer doesn’t work this way. In fact, this is a big part of its charm for those of us who love the sport. I loved soccer before MLS even existed, and I will love it even if MLS disappears.
See this post for some specific examples of how the sports has grown in popularity in the US:
https://www.cougarboard.com/board/message.html?id=10344786