There aren't a lot of games earlier than 1990 that I FREQUENTLY play. I'll occasionally play an NES game or even an old Atari game, but as for things that I replay every once and a while, I guess the oldest would be the original Alone in the Dark (1992).
I don't know why, but it has some sort of undying charm on me. Maybe it's because it was the first to implement a lot of elements that are now standard in my favorite genre (Survival Horror).
Sure, there were a few "horror" games before it, such as Haunted House on the 2600 or Sweet Home, but one was just moving a pair of eyes around a maze and the other was a crappy top-down RPG.
Maybe I just keep coming back because of Edward Carnby's moustache, which is, in my opinion, the greatest moustache in videogame history.