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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[b]Pillar of Autumn (CE):[/b] I like the whole thing where that level puts in motion the entire events of the game. The Covenant attacks and boards it which results in you escaping the ship to go to the ring and Keys lands the Autumn on it. Then we know what happens after that level. Now other Halo game has a first level which sets in motion the events of the game. The Pillar of Autumn is the only first level that has a cause and effect factor to it. I also like the whole thing where your ship gets boarded and you have to fight your way out thing about it.