Halo just brought the right components together to make a great multiplayer game. Of course it isn't the best FPS there is, because Bungie was smart, it made a game with EXCELLENT replay value. Think about how great Assassin's Creed is. Now, remember it's only single player. The replay value is zero.
Asherlee has a point. Playing Assassins Creed a second time is hard, my friend lost the game when he borrowed it and rebought a used scratched one so Im too disgusted to beat it a 2nd time.
Ah, even single-player games have replay value to me, if I enjoyed it the first time. I've played through GTA:SA about 8 times now and it's STILL got the epicness it did the first time. And let's not forget the people (like me...) who've played through Legend of Zelda on the NES so many times we don't even need to look at the screen anymore...