This is a great question! A few from the top of my head:

The original Assassin's Creed: I would never have the patience to play something like this on my PC. It is too repetitive and basic for me to enjoy while the rest of my PC is available. On Steam Deck though, I loved playing this in bed as a way to turn my mind off and just enjoy a simple story / game. I had a similar experience with the 2008 Prince Of Persia reboot. I 100% completed both, something I would never do on PC or TV console.

Undertale I also enjoyed way more during my second playthrough on Steam Deck. I think it has something to do with being fully immersed and again, playing from the comfort of my bed.

Also Slay The Spire, I got reasonably into it on a pirated copy on PC, then played it for hundreds of hours on the switch, then finally got a steam deck and it was the first game I bought for it, now sitting at 700h on Steam alone. So I went from piracy to actually owning the game twice :).

Any visual novel like Steins;Gate, Zero Escape series, Danganronpa... I would never be able to complete these games on PC or console.

What I notice, is that longer games mostly only work for me portably, because of the way you can sneak in extra hours on a portable machine. Time spent playing on TV or PC is always quite scheduled, and I often feel like I don't want to waste it on a single experience for too long.