My absolute favorite Design Hotels experience so far has to be The Guesthouse Vienna, which has since left SPG. Stunning hotel, excellent location by the Albertina and Wiener Staatsoper, friendly staff, lovely room upgrade to an opera-view room with window seat, four (!) complimentary full-sized bottles of Austrian wine, excellent food at the restaurant... you get the gist. A real shame it's no longer part of the portfolio.
That aside, the Rooms Hotels in Tbilisi and Stepantsminda/Kazbegi were both fabulous properties and real gems to have in the SPG portfolio with their characterful buildings, quirky idiosyncracies and zesty staff.
khabah