Original: Team Fortress
Team Fortress 2 was one of those games that seemed like it would never come out. Valve took their time with it, sure, but we learned ultimately that sometimes the wait is worth it.
The first Team Fortress game was simply a popular mod, and the sequel was at first released only in the "Orange Box" compilation with Portal and Half-Life 2. But despite those humble beginnings, Team Fortress 2 has a massive following to this day, over six years later.
And it was completely different from the first game; where the original was relatively realistic (for a Quake mod at least), TF 2 was cartoonish and goofy. Team Fortress 2 transformed the multiplayer series into a character-driven game of Looney Tunes-like action that's nevertheless well-written and even better designed. And it shows no signs of slowing down among its hardcore community of fans.