I would heartily recommend the Reval Ridzene Hotel. It caters mainly for the businessmen in Riga, although I found it as excellent choice for a hotel for pampering myself and my girlfriend. Having snaffled a great deal at www.activehotels.com I couldn't fault the hotel - central, quiet and well appointed. Being a businessman's hotel, it means that it tends to be pretty much empty at weekends - we found that we were practically alone when we had breakfast each morning and barely saw anyone in the hotel.
Sauna, comfort, great breakfast. The hotel has been constructed to very high Scandanavian standards - the Reval group is Scandanavian and there has been no expense spared regarding the fittings and furnishings of the rooms.