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. I wasn't that hyped for Halo 2 and 3, but I was excited. I was really hyped when I heard about Reach, but that died off because I was more excited for Black Ops. When my friend told me there was a special announcement at the end of Microsoft's E3 conference back in 2011, I guessed what it was (Halo 4), and he said yeah. I was, from that point until the release, hyped up for it.