Water.
:P
But the best effect is you know it's healthy, so you don't feel bad. Tea too. Green and white tea are high in antioxidants. Drink up.
2 weeks ago I would have said "coffee" - used to drink about 6-8 cups a day between 3 am and 7 pm (working hours). Over the past 2 weeks I've switched over to mostly water and 2 cups of coffee per day at most and its unbelievably fantastic.
Water for me has to be room temperature though. Not sure why.