Firstly, the taste of water: With something gross like Non-Diet Coke it always tastes exactly like an unhealthy sickly sweet drink. But water is different, water seems to change depending on what you have recently eaten: if you're hungry it fills you up like a delicious snack, if you've just eaten chocolate, the water will taste slightly sweet, if you've had something very salty it'll cool your mouth in a nice antidote kind of way. For all situations, water tastes the nicest.
Secondly, it's lack of calories: Unlike any other drink, no matter how much water you drink you won't get fat. I personally like to drink about four 500ml bottles of water a day at least, if I were to do this with Non-Diet Coke I would be getting about 800 extra calories a day and surely would end up fat.
Thirdly, it's always suitable: No matter what situation you're in, nobody minds if you have a bottle of water with you. You can't talk to a group of people while sipping on a can of coke, or walk through the shops with a bottle of Sprite. No matter where you are, nobody questions you having a bottle of water with you.
So I hope, I have rationally argued the reasons why water is my favourite drink. Lots of people find it strange, but this blog entry makes it pretty clear.