Sparkling wine Napa
Schramsberg - esp if you really want an in depth tour and knowledge of the wine. And wine quality is excellent.
I'd say if Schramsberg ends up not working out Mumm and Domaine Carneros are a toss up in terms of wine quality - but Mumm is better situated in a really picturesque spot off the Silverado trail.
Skip Domaine Chandon.