Halo isn't great because of its gameplay, but what it did. Halo CE, for example, was one of the first console games where people would set up LAN parties to play. While this has been done before, Halo was the first one to do it where it appealed to more than just the "hard core gamer" caste.
Halo 2 was one of the first major online console shooters with such a smooth online system. Before this, casual gamers couldn't really play online shooters as they would need to either A) Upgrade or obtain an expensive gaming computer, or B) play one of the less popular games.
Halo isn't great as a game, but its great for what it brought to the table; a solid, multiplayer console shooter. Anyone who says its the "Best Game Ever" probably just heard it from some techie who stated the above reasons and ran with it.