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 because of the atmosphere of the game