I dunno...for me its a contest between Halo Reach/Anniversary and Halo 4's armor. They seemed somewhat believable to me, and made the most sense.
For Reach's armor, the UNSC hadn't been in ANY wars except with themselves, meanign the insurrectionists. So they didn't have much need to really "change" their armor types or improve them. That's why they had Spartans; let the super soldiers do all the real work.
That's way, the covenant shows up, and then they are like, "Oh crud, they have plasma and such ,and we're still messing with bullets..we're screwed!"
Whereas with Halo 4, they looked as if they were trying to be Sci-fi as stated on the first page, but trying to look present dayish, since we all still use Kevlar and the like. But that's just my opinion.