I'll go with 999999.
Those numbers don't have any affect or impact on your taxes anyway. They're used mainly for statistical purposes, not only by the IRS.
I do know that in the past, if there wasn't a valid code that applied, then you use 999999. But I saw something somewhere fairly recently, that said to use 999000 instead. But like I said, I don't see how it matters really, since it does not affect on one's tax liability.
