I heavily disagree, CE's extreme repetitiveness and tedium and lack of content really hurts it. For reference, IMO:
2 > ODST > Reach > 3 >> CE = 4 > 5
2A > 2 = 5 >> Reach = CE > 3 >> 4
>Forge/Customs/etc
5 > 2A > Reach >> 4 = 3 > 2 > CE
2 > CE >> 5 > 3 = 4 >> Reach
ODST > 2 >> 3 > CE >> 4 > 5
Atmosphere
ODST >> Reach > 4 > CE >> 2 > 3 >> 5
5 > Reach > CE >>>> 4 > 3 >> 2
Note that on a purely mission basis, not counting story, i'd actually put 4's SP's over CE, as it does much of the same things, good and bad; without being as tedious; and that while 2 has objectively less custom/forge options, I actually had the most fun with it overall in customs due to the early xbox live community.
Reach obviously also overall has the most amount of content in every Halo game and gets bonus points for that.
I very much remember most the areas I had gone through, and can't remember any major scaling issue outside of perhaps the Library level. Otherwise, the 'decoration' for shit like the Pillar of Autumn was spot on, you ask me, and when it came to forerunner structures, left an air of 'ancient abandoned installations', which I quite liked.
I really disagree. When I look at locations in the other halo games, especially Reach and 2, the places feel like real places, whereas CE very distinctly just feels like locations designed as video games that have a very sterile, 90's/early 20's video game coordior feel to them that's plain.
4 does the same sort of atmosphere with abandoned rueins with synth music for them that CE does, but better, IMO, it's one of the few things it did right.