It's quite the experience, I started with the Index.
Unless Valve releases a new game with the frame I'd recommend trying Half Life: Alyx first, it's a very AAA-like experience and does a great job at demonstrating all of the capabilities of VR on top of being beautiful.
Once you've tried that Beat Sabre is a blast (and quite a bit of exercise). Superhot is probably the best selling VR game, you can live out your Keanu Reeves fantasy of being both John Wick and Neo. The art style ensures that you will have super smooth framerate.
VTOL VR is a flight-sim and a masterful example of creating a game using VR-only controls. It's for you if you like the idea of a game like DCS but don't want to spend 80 hours studying a textbook before you can take off (not to mention weapon systems and tactics) and don't want to spend a bunch of money on specialized control hardware. It is developed by one guy but allows custom maps so the community has created some interesting missions. There's even 2-seat aircraft (like an attack helicopter, training aircraft and electronics warfare plane).
Iron Rebellion looks like a promising team-based mech game using a similar VR-only control philosophy. It's in active development and the developers are very responsive to the community (it helps that the VR gaming community is small). It's hard to tell if it is going to be more Mechwarrior-like or more like Armored Core (though it is leaning towards the former)
Blade and Sorcery and Boneworks/Bonelab are physics playgrounds. Blade and Sorcery is more like a fantasy ARPG while Bonelabs is more sci-fi-esq. Bone* has mod support so there is a lot of user created content (mostly crap, but there are some gems)
VR Chat is pretty self-explanatory. It's not my thing but it is popular in some demographics.
A Fisherman's Tale is essentially an escape room that would be impossible to create without VR.
habie147 is a pretty popular creator that focuses on VR, I usually check his videos out before I try a game.
spoiler
There's also VR porn, if you see your Steam friends playing 'DeoVR Video Player' they're probably looking at sexlikereal porn.