I'm from the UK and I voted for fizzy drink, that's only because using a process of elimination it's the only one I would vagely consider using.
The way I see it:
Pop - Something you might say if you were about 5
Soda - Something you would say if you were American
Carbonated Beverage - Something you would say if you were a maths geek or a scientist or something.
Tonic - Something you mix with Vodka
Coke - That's a bit specific, you can hardly call a Sprite a coke now can u! That's just stupid!
Soft Drink - Well yes that is ok, although it doesn't specify if the drink is fizzy or not. You can have still soft drinks.
Therefore I am only left with fizzy drink (really only because it clearly describes what we are talking about)!:-)