Number two, singleplayer
Two games
As with the Number 5 position in multiplayer, we're tied here. Not only is it impossible to say which of these games is better, but it'd be just as difficult to try and bump one of the previous titles off the list.
Which is better? Which is more influential? Which is more popular? Civilization or X-Com - you decide. Civilization has spawned more college dropouts and more game designers that any other game before. X-Com is the Perfect Game.
Civilization is the pinnacle of Sid Meier's work, a simplified model of the world that somehow worked. It felt realistic in an abstract way. It was light-hearted yet serious. You had slightly goofy advisors and enemies, but ended up dealing with major issues like pollution. For some, the fun was in building up a network of big, advanced cities and leading a peaceful life, while racing to space. For others, it was all about being the game on Emperor difficulty, facing the overwhelming cheats and advantages the AI had at its disposal. At one point, you just weren't a hardcore gamer unless you'd beaten Civ Emperor.
Civilization influenced subsequent game design like few others. It was OK to abstract, diplomatic AI was important, war wasn't necessary, there was fun in research trees and building. Many would argue it's the best game of all time.
X-Com, however, is the Perfect Game. It did everything. There was absolutely not a single extra feature that was necessary. Replay value is limitless, as levels are designed on the fly and always new. The game has research, an excellent turn-based combat system, and all characters have statistics - an unheard of feature at the time. In fact, it wasn't until around the year 2000 that the trend to give everyone statistics and abilities became commonplace. But more than that, X-Com forces the player to defend his base, to capture aliens and equipment for study, to balance a budget, to build spacecraft and aircraft to shoot down the enemy. The player goes through a fundamental economic and gameplay change about one third of the way through X-Com, when he stops relying on funding from various nations and can support himself through the sale of alien artifacts.
Best of all, the game is perfectly executed. At this time, games were tackling the 640K barrier with odd methods, employing bizarre memory managers that attempted to trick EMS and XMS into behaving like basic RAM, due to limitations on MS-DOS and Intel processors which could only work with 1MB of RAM in real mode. We can see the effects of these managers in the problems that the two Ultima VII games had, for example. X-Com was quite bug free, easy to set up, worked with all major sound cards and graphics cards, worked with the mouse, and looked beautiful. There was nothing more anyone could expect. That's why it was, for a very long time, the highest-rated game by Computer Gaming World, at a time when CGW was the definitive PC publication.
For their lasting contributions and the sheer perfection of design in simplicity, we award Civilization and X-Com the tie for the coveted Number Two position on FiringSquad's Best Singleplayer Game List.