VLC Media Player, Media Player Classic, or Gom Media Player.
Those are my 3 top favorites for media players. All play most media formats out there, including FLV and DivX just fine. You shouldn't have issues with lagging or audio/video syncing.
VLC Link: http://www.videolan.org/vlc/
Media Player Classic Link: http://mpc-hc.sourceforge.net/
GOM Player Link: http://www.gomlab.com/eng/GMP_download.html
