Express.js seems to very popular to use as a general framework. Combined with socketio.js and underscore.js these are very popular libraries but they are not MVC.
For MVC I’ve only used backbone.js / spine.js and can’t give any recommendation for the others. I don’t think there is a framework with a dominating popularity going yet because node.js doesn’t have much use in production.
There are statistics for popular downloads with npm somewhere. See if you can find them.