Just a reminder that an onomatopoeia is basically a word which imitates the natural sounds of a thing. I am not sure what are the possible onomatopoeia associated with the popping out of a champagne bottle. I imagine there are a dozen, but I want the ones that are the most widely used.
You already said it - the best word is pop!
EDIT: J.R. points out that pop the cork is a very common idiomatic expression that describes the opening of a champagne bottle.