On the whole, the best Ramune flavor is Ramune original flavor taste or lychee.
Other popular varieties of flavors include watermelon, strawberry, lemon, and melon flavor. However lemon lime is a popular Ramune flavor.
The flavor of Ramune is quite unique, it has a strong flavor with a refreshing taste however, people from outside of Japan often find that it has a totally different flavor profile from say a standard soda like Sprite.
My own personal Favorite Flavor is the original flavor in a variety of flavors. It's the Classic Ramune Flavor or Japanese flavor and brings to life a sensation of japan whenever I taste it.