At the bar? Soda water/lime. Why? it signals to the bartender that you are likely a sober driver, and more likely a "graduate" of a 12-step program.  They will almost always give these for free, especially if you tip for the first one, regardless of whether or not they offer it for free.  There's definitely more psychology going on with this as well - but it all works in your favor, and makes you an instant ally of the bar staff, and can even allow them to be more liberal with your friends' drinks...