Quake1. Oh, you want me to justify this?
What do you say about Quake1? Practically the birth of competitive FPS gaming and certainly so via TCP/IP. One of the first games to make use of 3d graphics cards. One of the longest running online communities. A game engine whose code still survives in games put out today (
http://en.wikipedia.org/wiki/File:Quake_-_family_tree_2.svg). Also a game engine that had more revisions within Quake than outside of it (
http://en.wikipedia.org/wiki/File:Quake_-_family_tree.svg). The birth of tcp/ip free service game browsers (qspy) - as opposed to dwango and khali. The beginning of North America's largest lan tournament - QuakeCon. The birth of machinima. The birth of rocket arena. The birth of Team Fortress. One of the most modifiable games ever made with a built in c-code language called QuakeC. The soundtrack was developed by Trent Reznor of Nine Inch Nails fame. It runs on everything from Dos to Solaris to Sega Saturn. The source code was released into GPL in 99.
Blah, blah, blah. I'm tired of typing. Look, good games come and go, but games that are as revolutionary as Quake was are very few and very far between. That is the reason regardless of what game I enjoy at the current moment, I will always have to recommend Quake first.