Yeah. This one now.
Whilst PoA is obviously heavily iconic, and has surprisingly good design for a level built entirely of corridors, I'm vouching for Winter Contingency; I knew straight off the bat with this level that Reach's Campaign would be a stonker for gameplay and scale, and it generally was. Really enjoyable homage to Halo (CE's second level), looked beautiful too.
Honourable mentions for Cairo Station's zero-G sections and Dawn's brilliant climax and ending.
inb4best middle mission thread
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Mombasa Streets > all It's [i]easily[/i] the most atmospheric, varied, immersive and mysterious of the lot and was really well constructed. The only downsides to it IMO are the occasionally repetitive areas and lack of more things to find.