Quick answer
Tacos need what salsa already gives them: lime, salt and cold. A grapefruit or lime soda, sparkling mineral water with a big squeeze of lime and a salted rim, or a tart agua fresca like hibiscus (jamaica) or tamarind will match almost any filling.
If you can only pick one, make it a grapefruit soda over ice with lime — it is essentially a virgin paloma and works with beef, chicken, fish and veggie tacos alike.
Pair by filling
Carne asada and other grilled beef: grapefruit or lime soda, or a cola made with cane sugar. The char and fat want fizz and a little bitterness.
Al pastor: tamarind soda or tamarind agua fresca. Sweet-sour tamarind meets the pineapple and achiote halfway.
Fish and shrimp tacos: sparkling mineral water with lime, or a cucumber-lime agua fresca. Light, salty and cold, so the seafood stays the star.
Chicken and carnitas: mandarin or orange soda, or an apple soda. Gentle fruit sweetness suits milder, juicy fillings.
Veggie, mushroom and bean tacos: hibiscus agua fresca topped with sparkling water — tart, floral and great with earthy fillings.
When the salsa is hot
For very spicy salsas, sweetness and creaminess help more than extra acid: horchata (rice and cinnamon) is the classic cooler, and a mango or pineapple agua fresca works too. Ice-cold drinks and small sips beat big gulps of fizzy soda, which can make heat feel sharper.
Easy taco-night mocktails
Virgin paloma: grapefruit soda, fresh lime juice and a pinch of salt over ice, with a salted rim.
Ranch water without the tequila: sparkling mineral water, plenty of lime and a salted rim — dry, bracing and endlessly refillable.
Jamaica spritz: chilled hibiscus tea with a little sugar, topped with sparkling water and a lime wedge. Make a pitcher of the base ahead and add bubbles at serving.