I have to say, as much as there's plenty of gravitas for the criticisms of Halo being largely unoriginal, I still immensely enjoyed Combat Evolved. The lead up to the Flood was great, and once they bust out Martin O'Donnell's fifth strings bit in the escalating tension, it felt superb. Then just before the library, you see Elites getting messed up by the Flood, far out, that was great. For a super soldier, I quite liked that sense of impending threat, and playing the Library on even heroic difficulty was a tremendous challenge.
Getting the hell out of the Library for the first time was so great, the mad scramble down the hill through the jungle to the extraction point. Aaaaaaand then teleportation ex machina (sorry, I've been getting into Cinema Sins).