In terms of briefing tracks, the Title and Cinema tracks from FS1 take the cake.
As for mission tracks, my favorites are from FS1; the March and Spook tracks are by far my favorites. March has a sublime mix of foreboding, intense doom and badassery, while Spook laid the framework for one of the most memorable game sequences I have played. Used in 'The Big Bang' it made for a subtly tense atmosphere, but in 'Playing Judas,' infiltrating Shivan space with the low, mournful wails of space echoing in your ears and the driving drum beat and cello urging you onwards, soon to be joined by horns as more and more ships arrive, finally rising to a crescendo right as the node explodes with light and the Eva and Lucifer emerge. And there you are in your pitiful fighter, helpless to do anything except watch the Fall of Vasuda take its final shape. That really drove the 'you're all screwed' message home in a way only matched by the use of Lacrimosa to herald the Imperieuse's arrival in 'Delenda Est.'