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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Halo 4. After the disappointment that was Reach, I really thought that 343 would have learned from Bungie's mistakes and that Halo would come back strong. Boy was I wrong.