Water usually, occasionally with Berocca tablets (not sure I've noticed any energy benefits, but they taste nice).
A nice hot cup of tea with milk and sweetener at around 3 or 4pm every day - and no caffeine any later.
Diet Coke or Dr Pepper zero when I need to get some serious work done.
Starbucks skinny caramel macchiato when I want a treat, or their latte to wake up first thing. Occasionally Innocent smoothies as well, or homemade blends.
I only drink alcohol if it's free. This rule has kept me sober for months, but lets me get away with a glass or two when it's a networking event or dinner ;)