I'm going to have to say Half Life 2.

I was asked this by a close friend today, and as cool as Halo is, Half Life 2 just does it for me. Plus it was made by one of the most bad-ass companies of this video game era. I know some people out there prefer Halo, because lets face it, everyone knows what it is, and everyone wants to play it with their friends. This is just my opinion but Half Life 2 beats Halo down.