Whenever I'm traveling in a foreign country, I make a point to try the local tonic water. I do so not because I'm trying to cure malaria (tonic water's original purpose), but because it tastes better just about everywhere outside of the US.
Tonic water in the US, much like soda, is nearly always sweetened with high fructose corn syrup. And not just HFCS, but entirely too much of it. Much tonic water tastes flat and syrupy, and when added to a good gin it makes a mediocre drink at best. Worse yet is the stuff they serve off the soda gun at a bar.
In recent years, however, several new brands of artisanal tonic water have started appearing both in bars and on store shelves. These include the pleasantly crisp Fever Tree brand, the agave-sweetened and creamy Q Tonic, and the home-brewed flavor of the Fentimans brand. You can usually find these at natural food stores and at larger liquor stores.
These are so far superior to standard tonic water that I've stopped drinking the standard brands- or anything off the soda gun. Give them a try in a standard Gin and Tonic, or try something slightly more advanced like the recipe below.
By Jordan Silbert of Q Tonic
2 fl. oz. Gin
3 fl. oz. Grapefruit Juice
3 fl. oz. Q Tonic
1 dash of Peychaud's Bitters
Add all ingredients over an ice-filled highball glass and stir.