Distilled water about 95% of the time. Other than that; tea, coffee or whatever drinkable product I might have in the fridge at the time, like kefir or a smoothie for instance
Water. Safest bet while gaming. You don't need to drink too much to quench your thirst for awhile, and spilling some isn't going to make something permanently sticky/smelly.
Coffee, tea (if the game requires attention, careful reading of the author's text or characters' dialogs). Beer - if the game is a simple clicker or sandbox with repetitive actions.