Honestly, I'm expecting an optimisation disaster like what we've seen many times this year. I doubt it will run well on decently specced PCs (until the inevitable performance patches drop), let alone the Steam Deck.

It's gotta be DOOM! The first game to introduce me to PC Gaming outside of Minesweeper and Solitaire!

SkiFree, I thought it was a serious simple sports game until the Yeti appeared!

Other than this, DOOM/DOOM2 for sure! They were the first games that made me feel like I was actually there and part of the action.