Most treasured possession (if property is allowed) is my 15th century house in the centre of one of Europe's most beautiful cities (and UNESCO World Heritage Site), Torun in Poland.
Link to huuuge picture:
http://www.wrzesinscy.com/wp-content/gallery/torun/torun%20panorama%20stop%20v2.jpg
If I had to limit myself to more portable things, I'd probably say my Leica IIIc, or (and this is actually still on order so not currently in my possession at all) my Stowa Marine Original wristwatch.