DOOM (via UZDoom)

Full of surprises from the start. Even in the menu it was kinda fascinating to see that such an archetypal capital-G Gamer game has a very easy mode. Other highlights: the ways you can feel the friction of the devs grinding their minds against the technology of the time (e.g. the whole 2.5D planar levels thing); secrets within secrets; how the use of simple lighting effects and darkness adds so much tension and switches up the pace of the game; and how good they got movement feeling already in 1993. It was also interesting to see proto-Quake aspects in some of the later levels: more monumental designs, varied level layouts, and recontextualising places you've already been.