im going to have to say Monaco of the ones listed.......of all time:
Monaco, Monza (and the old 10k config), Spa, Brands Hatch, the old Watkins Glen before nascrap got thier hands on it and fcked it up, and of course, the Nürburgring Nordschliefe with the old start/finish pit strait (and the Südschiefe)..........basically, the classics from the 50's and 60's.......