I mean, sure, Reach was a button-pressing, plot-light military-focused grit-fest. But it was also a technical marvel, with Bungie's first ever space flight level in a game (and the only one to this day), and unprecedented sense of scale and emotion for a Halo title.
I understand that ODST had likeable characters compared with Reach's, but the plot was only slightly less dull than Reach's, and level design was standard Halo fare, nothing truly amazing besides maybe Coastal Highway. Don't get me started on how badly Bungie failed at the open world, something that has infamously continued in Destiny.
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By far the Jazzy scores by Marty, the rainy nighttime sneaking, the detective style arc... Man it was the best campaign for me. Yea, killing an endless amount of baddies, traveling across the galaxy, and saving humanity is cool, too... But man, ODST :).