I voted France, Germany, Belgium and Sweden.
France of course, due to the sheer variety of our regional styles but chiefly because of our inexhaustible inventory of Gothic buildings.
Here an example you've certainly never heard of (most French aren't even aware of its extence), a parish church serving a village of 650 inhabitants less that 200 km East of Paris, Notre-Dame de l'Epine:
http://upload.wikimedia.org/wikipedi..._l%27Epine.JPG
French Gothic is not only churches, see the Pala of Justice of Rouen:
http://his.nicolas.free.fr/Histoire/...stice3_WEB.jpg
Germany because of the wonderful Hanseatic brick Gothic (Lübeck, Dantzig), and also because of its revival als Backsteinexpressionismus in the 20's:
http://upload.wikimedia.org/wikipedi...chhaus-HAZ.jpg
http://upload.wikimedia.org/wikipedi.../Chilehaus.jpg
http://upload.wikimedia.org/wikipedi...rstrasse01.jpg
Belgium because of the well-known Flemish Mediaeval and Renaissance masterpieces, but also because of the less famous Mosan Art of Wallony:
http://upload.wikimedia.org/wikipedi...anvier2006.jpg
http://www.grandcurtiusliege.be/images/curtius2.jpg
And Brussels hosts one of the most revolutionary buildings ever, the Palais Stoclet, built in the years 1907-1911. With it the Austrian architect Josef Hoffman literally invented Art-Deco, well ahead of its time:
http://upload.wikimedia.org/wikipedi...17_%281%29.jpg
Sweden because I'm madly in love with Nationalromantik architecture, an impressive endeavour to develop a uniquely Swedish style (basing upon late German Jugenstil) in the early years of the 20th century:
http://upload.wikimedia.org/wikipedi...ö_högskola.jpg
http://upload.wikimedia.org/wikipedi...ity_Hall_2.jpg
http://upload.wikimedia.org/wikipedi...runa_kyrka.jpg
http://www.ostersund.se/images/18.2e...radhuset01.jpg
http://www.aviewoncities.com/img/sto...kvesv0536s.jpg
I'm not crazy about Italian styles, except for Fascist Neo-Classicism of the 30's:
http://www.imageschangetheworld.com/...y/_DSC9060.jpg
Florence's S:ta Maria Novella (mid-30's) is the most perfect railway station ever:
http://www.tuscaning.com/img/art/67-...4947182_l.jpeg