Belgrade is an amazingly eclectic and fun city. I feel sorry for the local tourism scene that it's not more well known and visited because there really is a great vibe about the place.
Square nine is still my favorite, the Concierge Nikola is an absolute gun. I arrange everything through him from guides and transport to club bookings etc he has the contacts. The hotel itself is located in the downtown area, appointed with lots of antiques and art. It's very easy to get around, the food and beverage is excellent too.

St Regis is good too; however it is located in the newer river front area. It doesn't have the same homely style as square nine.

Enjoy Belgrade, if you get a chance go for lunch at Fleur de Sel at the Sapat Winery. It's a nice day trip out into countryside by the Danube.