Basically, all of the Valve ones. (Half-life 2, Half-life 2 Episode 1, Half-life 2 Episode 2, Team Fortress 2, Left 4 Dead, Left 4 Dead 2, Half-Life Alyx)

Each one of them is a time capsule of the techniques and game design philosophies at the time of development. Pretty sure there is a walk through on youtube for every single one of them, if you don't want to get the games yourself.