I have an address in the USA but live most of the year overseas. Which address should I use?

You should use the address where you will promptly get any mail that is sent to that address. The address on your tax return is a mailing address. If the IRS needs to contact you for any reason (including sending a refund check) they will send mail to the address on your tax return.

IRS notices give you a limited amount of time to respond, typically less than 60 days. If you don't respond by the deadline you could lose the right to challenge an IRS change to your tax return or other IRS action. So you want to make sure that you will receive any mail that is sent to the address on your tax return without delay.