It's gotta be Halo 2 for me. While I was still at the absolute nadir of my interest in gaming, Halo was the biggest part of my gaming experience in the first half of the 00s, and Halo 2 was a big deal for me. It was the first game I ever went to the midnight release for, and while I didn't like it as much as Halo CE, it was still an incredibly fun experience. The LAN days did give way to playing on XBL, and there was problems with cheaters on that, but several of the people I still play online with today I first met back on Halo 2. I still play the Campaign missions every now and then.
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