Personally I think Reach is Halo's weakest entry from a campaign standpoint, both layout-wise and story-wise. I enjoy the weapons and enemies well enough, but exploration just wasn't that fun compared to past (and future) entries in the series.

ODST on the other hand is one of my favorites layout-wise, and I'd love to see that kind of "open world" Halo gameplay revisited one day.

343's entries aren't perfect by any means, but I really like 5's level layouts and weapons, and although the enemies lacked in 4 I still thought the campaign was fun. Despite the lackluster stories of both titles, I don't think they were much worse than Reach. Character development-wise, I think 4 is probably still the best for that specifically; it's the first time we really see Master Chief consider himself as a part of humanity and not a weapon for humanity, and it took an AI to help him reach that point. I liked that.