The Odeon is far and away the best cafe in Zurich, although the atmosphere varies a great deal during the day, from a quiet, gentleman's club-like atmosphere in the morning, to a busy after-work drinkers' buzz at six o' clock, to an almost-nightclubby kind of groove after ten.
If it was good enough for Mussolini and James Joyce, it might be good enough for you...
http://www.odeon.ch/de/