Halo CE - First fight against the Flood
Halo 2 - seeing The Gravemind for the first time
Halo 3 - the final drive to the Dawn
ODST - roaming New Mombasa at night
Halo Wars - watching the cut scene with Forge, Jerome and the other Spartans kick some Covy ass defending the warp drive
Reach - defending the Pillar of Autumn and watching the cutscene