Asklemmy

Are we assuming the drink magically has no calories or other negative effects?

I'd choose white wine if it doesn't get me drunk (and as long as it still tastes good and not shitty like alcohol free wine. And also as long as there's no social repercussions from drinking wine all day.)

If the drink keeps all it's normal properties then I'd choose green tea.

Decaf tea. Though that's just water with a sprinkle of leaves. Not so different from a twist of lemon or a slice of cucumber.

I'd say Coke Zero, but technically this is just a "water with some chemicals in it" goof, and therefore not allowed.

Whole milk

Man, it had want to be a HUGE drink! What happens if I run out?!

Birch sap is my first thought, but I might get tired of the sweet. Some kind of tea then, maybe rooibos. It's sweet or savoury as the occasion demands.

Some herbal tea. Mint would probably be best. It's what I drink most of the time anyway.

Unsweat tea

It has to be refreshing, not too sweet. Because you'd need to drink a lot every day to stay alive, off the top of my head rice water sounds like an ideal drink, I love the flavor and it has less sugar than horchata, you don't want to have to stuff yourself with sugar every day when you're thirsty.
Another possibility is hibiscus water. Again - if I could regulate the sugar levels it would be a way to go.

Cider

Ayran.

OG Four Loko

Odourless, tasteless, colorless, uncarbonated soda pop.

lacroix boiii

I've been drinking brewed cacao (Cacao beans roasted, ground, and brewed like coffee) almost every day lately. So that.

H1O mixed with some Hydrogen. Check mate.

Go big or go home, I say. Pappy Van Winkle for the rest of my days.

Coca Cola. I love it, but I try not to drink it too much. A nice iced can of Coke every now and then.

Is aforementioned drink free for life too? Because there's this whiskey distillery just up the road from me and they make a black tea infused whiskey that I wouldn't mind.