Recently I've moved back to VLC, the player has improved substantially in recent versions over older versions. A personal "required" feature I need is a decent minimal interface view, with the video taking as much screen as possible while keeping the taskbar on screen.
I used to use MPC (Media Player Classic) for this purpose, however, I've had display issues using the minimal view for that player on Vista with Aero enabled. It still technically worked, but left the right-side of the window displaced enough to show the entire border and a bit of the desktop.
VLC apparently recently added a minimal view, which works quite well.