Borders
Should the United States government open its border wider, close them tighter, or continue its current policies? You might be surprised that many anarchists have very strong opinions on the matter. This argument is especially decisive amongst libertarian anarchists. And that is the problem.
Anarchists debating what border policy a government should pursue is akin to Christians debating which of Satan's proposals are better. In the end it doesn't matter because Satan wins. Likewise, it doesn't matter what border policies a government pursues because the State will win.
Instead of debating what the government should do, anarchists should be pointing out why the government shouldn't be involved in borders or anything else.