Since I prefer controller over mouse and keyboard, it’s a lot of games pre-Xbox One era for me. But that is subjective because a lot of people prefer mouse and keyboard.
Games like Oblivion never had a proper way of getting a controller to work with them on the PC release and any tries to get it to work flat out suck. I’ve tried Steam’s button mapping and even that doesn’t work for a lot of these games. But working through an emulator helps translate my controller to the game with almost no issues.
Considering how the Steam Deck plays and you’re usually using it as a controller unless docked, this would be nearly any game from this time period.