For me, the first one is still the best, and the only one where I've played the campaign multiple times, including beating it on Insane. Also, playing Warzone/Execution was awesome especially when you had 6-8 buddies to play with. It was really social, it wasn't about ranking up and leveling up or whatever like the subsequent games became. Sure, it didn't have Horde and that was an amazing addition to subsequent games, but overall I still prefer the first game.
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