The best virtual reality game, probably.
Beat Saber is a rhythm game in which you swing lightsabers to slice through blocks to the beat of the music. The blocks need to be sliced with the matching coloured saber and in a specific direction indicated on the face of the block. If you've played Guitar Hero or Rock Band, you understand the fundamental satisfaction of this, but slicing objects in VR amplifies that pleasure tenfold.
The game's initial tracklist is good, though I'd recommend picking up some of the DLC packs as well. My favourite is the Panic! At The Disco set, in which I've spent too many hours perfecting High Hopes on the harder difficulty settings. I'm not great at rhythm games (or games, generally), but Beat Saber offers plenty of options to help me practice, including turning off failure states and playing tracks from any start point and at any speed. It feels great to replay a chunk over and over at gradually increasing speeds until I've fully mastered it, and then to put together with all the other parts of the song.
I'd be more confident in calling Beat Saber the best VR game if it wasn't Half-Life: Alyx. They are so different as to be incomparable though, so let's call it a tie.