youtube.js

(function() {
    var formats = ytplayer.config.args.raw_player_response.streamingData.adaptiveFormats;

    formats.sort(function(a, b) {
        if (a.width > b.width) return -1;
        if (a.width < b.width) return +1;
        return 0;
    });

    window.open(formats[0].url);
})();