If I had to pick just one, Naver Map. It’s easy to handle Korean address and place searches, public transit routes, and walking directions all in one app, and it supports foreign languages too, so it’s a solid choice for finding your way around on the first day. That said, information can vary a little between map apps for different places, so it’s a good idea to check KakaoMap too for restaurants or cafes.

After that, it depends on how they’ll get around. If they’ll be taking taxis, I recommend Kakao T; if they need to read menus or notices, I recommend ​Papago. For payments, first check whether their card can be used overseas, and for transit cards, a physical card like T-money, which you can buy at the airport or a convenience store, is still convenient. Korean mobile payment apps sometimes don’t let you sign up with a foreign card or phone number, so it’s hard to list one as an essential app from the outset.

My order is Naver Map → Papago → Kakao T. I guess the priorities will vary depending on which country my friend is coming from and whether they have a Korean phone number or payment method.