I use Traktor Pro 2 with Native Instruments Soundcard for Audio playback over ASIO. It is a DJ-program with 32-bit floating point audio engine and realtime processing. It was not easy to configure, but sounds really better than every mediaplayer I heard and no lags :-)

Definitly incredible sounding Player for flac is the DJ Software Traktor pro 2 from Native Instruments. With an Xonar Essence and WASAPI is there nothing better I know.