I just remembered that Transport Tycoon was 1994. It's a grave injustice that that isn't a poll option. Like X-COM, that game is revered and considered unequaled 24 years later. As much as I love X-COM, I would have to think long and hard about that. Maybe this is for the best, though, so that I don't have to choose between them.
Man, 1994 was such an epic year for PC gaming: X-COM, Transport Tycoon, X-Wing vs TIE Fighter, Master of Magic, System Shock, Warcraft: Orcs & Humans, Doom II, Heretic, Magic Carpet, Wing Commander III, The Need for Speed, Theme Park and no doubt more that I'm forgetting.