What's the smartest way to use a tax refund to attack credit card debt?

Apply it to your highest-interest debt first (the avalanche approach), since that's a guaranteed return equal to that card's APR, often 20%-plus, which no safe investment matches. A lump sum is powerful because it knocks down principal directly, immediately reducing the daily interest accruing against you and lowering your credit utilization, which can lift your score once it reports.

A sensible split: keep a small slice to shore up a starter emergency fund if you don't have one, so a surprise expense doesn't send you right back to the card, then throw the rest at the debt. Also consider adjusting your W-4 so you get a smaller refund and more take-home pay each month to attack debt continuously rather than once a year. See the payoff impact at wealthserene.com/tools/debt-payoff.