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Halo is still a pioneer in many regards in the FPS genre, but its true beauty is in its mythos and universe. It's so rich and full compared to so many games, even typical RPG style games. Read the books, comics, whatever. Watch any of the shows (the collective of animation stories that came out awhile back is VERY good for story.) As for the games, Halo Reach and Halo 4 kinda went astray from the classic formula to try and be more like modern shooters (i.e. COD and Battlefield) in terms of gameplay, but they did maintain their own unique, Halo-esque brand. Something with Halo though is that the campaign is actually commonly the focus for a lot of its fanbase, which is rare for a game in the genre.
I would also like to mention: never played Half-Life. Could never seem to get myself into that series, despite trying very hard to do so.