No, for Cro it's either a Crémant de Bourgogne, a Crémant d'Alsace, or even a dry Italian Prosecco.
I do, of course, keep a couple of bottles of Mumm in the fridge just in case Wills and Kate should pop by, but it's something we would hardly ever open just for ourselves.
My most recent favourite fizz is this l'Altissime (foreground). It has no appellation d'origine on the label, but is extremely good. The other one in the photo is a good Crémant d'Alsace.
This may sound like heresy, but the important things to me about fizz are its taste, the appearance of the bottle (v important), and the cost. A bottle of l'Altissime costs me about €6 (under £5) and when chilled is perfectly delicious. Unfortunately a bottle of very ordinary Champagne would set me back at least 3 times that, and would probably be unremarkable.
We might have a bottle of Mumm for Christmas Day, but otherwise I'll stick to the Crémants.