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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I dunno, I've just become so accustomed to speedrunning the sh1t out of PoA... But I'm gonna say Mombasa Streets, purely from an atmospheric point of view. In terms of gameplay and pure nostalgia, PoA. And in terms of glitches, well, it doesn't really get much better than Cairo Station.