Good old Coca Cola is my go to. When I hit up the Taco Bell I like the Mountain Dew Baja Blast.
Check out your local Japanese supermarket. In Japan they have all these fancy (but expensive) sodas and ramunes (the little bottles with a marble inside). My favorite is the Melon I guess. They have peach coca cola, lime coca cola, matcha tea, etc...
Oh yeah I almost forgot MEXICAN COCA COLA is actually my favorite soda, it tastes different than the USDM Coke the sugar is different. Would always get it when we went to Baja but you can get it at a Mexican supermarket.