Any country that you will visit your best friend can be fun and exciting. You and your best friend understand each other perfectly. Even if there are some things that you do not like about each other, you know that your bond is so strong that you are going to have fun no matter what. Going to a place that you both like is always ideal.
You can discuss your top 3 options with each other and decide on the country that you both like to visit based on the activities that you will do, the period when you are planning to go, and of course, your budget. You need to have enough money to be comfortable throughout the duration of your vacation.