Definitely the most underrated Halo game ever. I just beat ODST on legendary, not only to get a whopping 300 GS, but also to revisit the game. I have to say, I haven't had that much fun in a video game in quite a while. The story was just so great, and the characters really made playing the game an awesome experience.
ITT: Post what you love about Halo 3 ODST
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Everything. Best campaign in series. Halo 1 is close second, but for first time playthrough my best experience was ODST.