Bungie-era Halo has the best OST in the industry hands down, though recently Lunacid let me hear some certified bangers; someone here mentioned B:G&E, and I second that opinion although I wouldn't have thought of it myself.
The use of language in Reach is masterful. Even with one liners like 'DOT, check your vector', or mutterings like 'armour configuration matched' - 'shield strength too' add so much believability to the world.