Call of duty 4
Gears of War 2
Halo 3
Halo 3 was the first game that I played on Xbox live and for a long time it would have been on the top of the list but after comparing it to these other two games I have realized that Halo isn't the only good shooter out for the Xbox.
Call of duty 4 - I love the modern, more realistic sense of combat and the weapon customization. The weapon customization is one of my favorite aspects of the game, because I tend to like to switch guns around almost every time I die (if I do die). Only downside is the lag and how it ends the game if a host leaves instead of finding a new one.
Gears of War 2 - I didn't play much of Gears of War 1 but after loads of convincing from a few of my friends I tried out Gears of War 2. Gears of War 2 was a whole new experience for me as I have not played altogether too many third-person shooters. As I progress through the campaign, I looked back on Halo 3's campaign and realized how much better Gears of War 2's campaign was. It was a lot more dramatic, however both games had lame end boss fights. The only reason this game is above Halo is because of the campaign, I never got into the multiplayer all that much.
Halo 3 - This game was great for a while... But like Uniflash says, you don't find as many helpful or inviting people in that game. It's almost completely lost it's team aspect unless you're playing with some friends who know what they're doing. The forge and theater function and custom games are a great idea, but they tend to get boring after a while. Don't get me wrong Halo is a fun game, but next to Gears of War 2 and Call of duty 4 they're nothing.