Huge soda fan so picking just one is super hard but I’d say:
Oronamin C or Sweet Blossom’s Rose soda
Oronamin C is a Japanese soft drink with a taste that is often described as “drinking (American) Smarties”. It’s incredibly refreshing and delicious. Sweet Blossom is I think an eastern European beverage company and they make a rose flavored soda that is to die for. It has far and away my favorite quality of carbonation (very fine, soft bubbles).
Other honorable mentions include Ocean Bomb: Sailor Mercury (i.e. pear flavor), and as a big fan of cherry colas, Bawls Guarana Cherry Cola and Fentiman’s Cherry Tree Cola.