Every currency that's backed by an actual government. Whatever else happens, you can always pay your taxes with it. It is the currency of the economy the currency is native to. As long as that economy is running, you can use that currency to purchase goods in that economy, and the currency has value. People doubting whether enough people will start using it in the future is irrelevant, because enough people are already using it in the present. A country will have to work really hard to sabotage its currency, and that's extremely rare (though it has happened in Zimbabwe and Weimar Germany).

Bitcoin, by comparison, isn't used for everyday transactions by a lot of people (though there are some). There's no real economy it's native to (unless you count Silk-Road-style markets), and there's no guarantee that you'll be able to use it there in the future.