I'm going to out my HN account to my friends if they're on here, but I've been playing Noita, Vault of the Void, Vampire Survivors, Heroes of Might and Magic, Bastion, Boneraiser Minions, Distance, Pawnbarian.
Huge range of games with a wide variety of inputs. I don't want to admit to you what my weekly playtime average looks like since receiving the Deck, but it's high.
Anything that has natural stop-off points pretty much.
Vampire Survivors is my most highest played one based on hours though =) You can play it with the sound and most of your brain off, so you can still listen to what's going on in the TV show you're "watching".
GTA V runs perfectly on it, I played through Quantum Break. And as soon as I get Heroic Launcher (Epic + Gog games) working, I'll start with Witcher 3 and CP2077.
I've been appreciating it for games that don't have a natural stop-off point. Unlike on my computer, I'm perfectly content to pause the game, sleep the device, and then pick it back up again later. The ability to turn it off without losing your state is a given on devices like the Switch, but it feels like a really important accomplishment on a PC where I'm often hesitant to even sleep the machine while I have a game open.
I've been playing a lot of Vault of the Void, which is honestly mostly a mouse-only game on the Deck, but being able to map a few key keyboard shortcuts to the plethora of buttons makes it feel effectively the same.
Honestly, its more that it feels like a new input type entirely in a class of its own. I find I use it a lot to mix and match gamepad controls with keyboard shortcuts and mouse click-drag, sometimes all within the same game.
Noita has gamepad controls, but a lot of the interface works a lot better with mouse+keyboard. Deck lets me use gamepad controls for when my character is interacting with the environment, and use mouse+keyboard controls when I open my inventory. This wouldn't be realistic on another device, because the environment/world continues in the background even while you have your inventory open, so you need to be able to switch immediately from inventory management back to world-interaction.
Most games are like this: they rely heavily on one input, but being able to make use of the other inputs simultaneously gives you a ton of options.
The one class of game that I really enjoy but haven't tried yet is heavily-keyboard-focused games like some traditional roguelikes. Some, like Brogue, have a small enough control set that it would be pretty easy to map all the main keys, and supplement with mouse input for eg inventory selection. Many require more keys and don't allow mouse input, however. Cataclysm: Dark Days Ahead has something like FIFO keyboard shortcut for inventory items, and uses a-z plus several symbols; that would be pretty painful on Deck. I haven't played that in years though, so maybe would be better today. I've been seriously meaning to try Cogmind or Caves of Qud on the Deck, though.
It's my favorite airplane activity, but it is really extremely limited compared to the PC version. It has all the vanilla game components but big cities struggle. After a certain population amount the game removes the 3x speed option.
Personally I'll always prefer mouse+keyboard, but it's also fun to play games in the living room.