Andrea Riquier What’s the best investment you can make in 2018? Three considerations—taxes, risk and the economic cycle—point to one conclusion: Paying down debt looks like an awfully smart move. Debtors’ prison. Ridding yourself of debt, even mortgage debt, has long been a savvy alternative to buying bonds and certificates of deposit. But thanks to the new tax law, it looks especially savvy right now—and especially if you’re married. How come? The new tax law took away personal exemptions...