Solo, it depends on the day.
If you love Art Nouveau, the Smetana Hall, mosaics and Mucha's Mayor's Hall are the most ravishing interiors in the city, best seen on a guided tour or by attending a concert. The catch: the lavish rooms are tour-only on a set schedule, so you can't just wander in; the café and exterior are free but the real splendour is behind the ticket.