I think it's obvious to all but the most devoted Halo fanboys that games like Halflife (1+2) or Farcry have a better single-player game.
However, the one real true point that Halo has over most other games is the multiplayer experience, which is, frankly, a blast, especially compared to other games where it seems like the multiplayer is tacked on. Grabbing 3-15 friends, having access to a basement and 10 hours on a saturday made life worth living. Especially with Halo 2, where multiplayer rewarded players for knowing the map intimately, i.e.: "secret" sword locations, jumps onto the tops of buildings, etc. And that whoosh sound on the plasma sword was just designed to make people scream, "Where did he come from?!" or "Consarn' it!"
And even those who do not have Xbox Live still appreciate a quality multiplayer experience.