Think there was a long thread on this sometime back
Personally I use KM Player for upto 720p
And MPlayer Home cinema classic (that uses SM Player as the front end) for 1080p
KM Player because I find it the most flexible and efficient player which allows quick access to any sort of setting including enabling/disabling video/audio filters and sub resync (time /framerate) with keyboard shortcuts
(I watch a lot of foreign language flicks and this is an essential feature for me)
It also allows me to route DTS/DD audio untouched to my AV receiver optically, while decoding other formats to PCM
SM Player/MPC Home cinema for 1080p because this is the only player that can efficiently handle DXVA (hardware decoding on GPU) for x.264 (most mkv's we get are x.264) thereby allowing playback/seeking without any AV sync problems
Now SMPLayer/MPC can also handle most if not all of what KMP can, but accessing the features is a lot more convluted