Halo 1 began an imagination land of FPS. Regenerative health, vehicles that were awesome, a space story that wasn't lame and a sense of exploration onto truly crazy planets. The original Halo gave us proof that if you let creative people build games, those games will find meaning and a fan base that will chant for more. There was no way a Halo 2 would not be born and when it came out it was brilliant.
Halo is still played amongst my friends. We have launchers on flash drives so we don't even have to install the program anymore. Its multiplayer combat as well as its phenomenal exploration into the final hours of peace among humanity and the covenant gave it wings.
Halo 2 built on that and made such great progress that many people will still play it saying its better than Halo 3 while the newer Halo fans on Halo 4. Take what you will from all that, all we can be sure of is that Halo is a loved experience amongst single and multiplayer gamers and it will continue to be as time goes on.