Final answer:
The best feature for a credit card used only for emergencies and paid in full monthly is no annual fee because APR and rewards are less significant in such usage.
Step-by-step explanation:
Given that you plan to use this credit card only for emergencies and intend to pay off the balance in full each month, the most important feature for you to consider is having a credit card with no annual fee. Since you'll be paying off the balance each month, the APR, or the Annual Percentage Rate, becomes less significant because you will not incur interest charges. A generous rewards program and no balance transfer fee are less relevant if you're primarily focused on emergency use and not on earning rewards or transferring existing balances.