Everybody would be using VLC here but I like Media Player Classic - BE just because of one killer feature - seek bar video preview!
MPC-BE is an open source video player based on the original Media Player Classic (MPC) project. Similar to MPC-Home Cinema, MPC-BE contains great features and configuration but the best feature is it's seek bar with video previewing. You have seen this feature on YouTube. You just have to hover your mouse cursor over the seek bar, and the player will show you a frame from that point in the movie.
You can enable this in View > Options > Interface > check "Use the preview in the search".