If I could only have one, it'd be VLC Media Player.
It's a fast, lightweight app, rock solid, and plays everything I've thrown at it where other media players have failed.
But I would really like it to have a 20+ band EQ with adjustable filter frequencies and width, and to save EQ settings when closed. A ten-band EQ with set filter frequencies and width, that resets every time I close the program, is not cutting it. Maybe there's an extension/plug-in out there for this ... time to search.
Also, I prefer Windows Media Player's layout with the playlist docked next to the video/visualization, rather than the separate window (by far) or the docked option where the playlist still isn't next to the video. Same for Windows Media Player's "Enhancements," back when they were docked under the video.