Quake 1 (CTF + Retextured + New Lighting + New Models)

The first real internet mutliplayer game. Since the release of the source code there have been many improvements to the game engine to improve the quality of graphics, while keeping the game play the same. Quake revolutionized first person shooters and still remains somewhat active 10 years later. Qrack is the latest engine to take advantage of new textures and improved graphics while maintaining the gameplay.

http://en.wikipedia.org/wiki/Quake#Modification

Huge 1 on 1 tournaments and vastly popular CTF made this one of the best games of all time. All the Quake sequels haven't come close to matching the speed, weapon balance, and skill this game entails.

Quake 1 invented Capture the Flag as it is today and it started the idea of the grappling hook. New versions of Quake do not use the grappling hook in CTF. The Death Match in this game is amazingly complex and unrivaled...The idea of clans was generated from this game. This game still has many active players and those seeking some fun shoudl definately try this out.

While the game is old, with the improved graphics, it rivals the best games out there today.