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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Pillar of Autumn was an awesome level to me. Although all of them were amazing in thier own way. Perhaps Pillar of Autumn stands out because that was my first Halo expirience.