Call of Duty 4 or Orange Box, Halo's novelty wears off quickly. While CoD4 is a faster paced shooter with weapons from the present, it's very exciting.
The Half Life series is just EPIC!!! Half Life series just blows Halo out of the water no matter what. I'd say that no shooter series even compares, and orange box also comes with the revolutionary game Portal, and the multiplayer awesomeness that is Team Fortress 2. Why would you pass that up? I have all these games and in my opinion Orange Box and CoD 4 take it, I have timeshift but I have yet to even crack open the box, so I can't help you there.