If just out of sheer volume of consumption, it would be Diet Coke. I usually have one in the morning before work, one on my first break, one with lunch, and one for the drive home. That's about 80 ounces of that carbonated stuff. I'm sure my bones hate me much for all that abuse.
First runner up is water, and second runner up is 2% lowfat milk.
Now, when it comes to cocktail hour
I usually tilt back Smirnoff Ice like it was lemonade. Wait, come to think of it, it really is lemonade! A touch more potent, but lemonade just the same!
My other favorite is Bailey's Irish Cream. Anything made with Bailey's Irish Cream!
I've also been known to try different concoctions just to see how they taste and make me feel. But the key to my drinking has always been moderation. I never drink more than three hard drinks at any given time. If I do, I will not leave the establishment I'm drinking at for two hours after I've downed them.