My favorite would probably be No Man's Sky. I had known about it for years but never tried it until this year. I was completely unprepared for how much I would get into it, and how much time I would spend in it. It worked so well on deck too, being able to put the deck to sleep, and then wake it up hours later to collect the harvest from a plant or mineral extractor was very convenient.
My most disappointing was Diablo 4. I was really glad to see it come to steam and thought I would love it, but the always online requirement and how they handled disconnects really killed my enjoyment of the game. One of my favorite things with the Deck is being able to suspend my game anytime and pick it up later, but suspending the deck with D4 means it will disconnect and I'll have to restart the game when I return. It wouldn't be so bad if I just had to wait on the game to reconnect or something similar. It's also a game where you can't pause, and setting it down with the game running always seems to result in my character getting killed by some monster that wandered too close and agro'd. Some other games with always online requirements (like Honkai Star Rail) handle internet much better, I can suspend at anytime and just have to wait for it to reconnect when I wake it back up.