For me, it was Halo 4.
I thought it was going to bring back the classic Halo that I liked. Honestly, I wanted it to be a Halo 3.5. That hope was quickly crushed by the return of armor abilities and other crap.
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easily halo 2, i mustve been in like 5th grade back then, and i still played halo:ce at the time with all my neighborhood buddies. also im pretty sure i brought halo 2 in to class for show and tell the day after it came out