I usually don't play sequels before the originals, its my OCD. (same with movies/shows/anime/anything) There are a few that I did thought:
Fallout 3: the other games are too old for me to bother with.
Oblivion: same as fallout 3, as soon as I heard Morrowind has no fast travel I knew I was never going to play it. ( I also heard the fastest way to travel was to continually jump, lol)
Metal gear solid 4: I wanted to play this game, but I didn't have a ps2 so I could not play the other 3. The railgun is what I love about game, I don't care if there is a better weapon. (iv been hearing rumors of a combo pack of the first 3 games, any one know anything about that?)
Prince of Persia Warrior Within: The first prince of persia game I played, it was cheap and I didn't that it was a sequel. I now own the main 5 prince of persia games, and love them to death. By main five I mean the trilogy, the reboot, and that one movie tie in.
Halo 3: it came with my xbox, been meaning to get the other 2. (for the past 3 years...)
Note: COD is weird, its not really sequential from what I can tell, except for MOD1/2. My first MOD disk is scratched since I got from one of "friends" from school so I can't get past the third mission. I did finish the second one, but the story went over my head. Something about Russians and terrorists...
Rayman 2 the great escape: got it my n64, this was when I just started gaming.
Half-Life 2: I played the episodes in order, but haven't played the first one. I haven't seen many people talk about the first one so I assumed not many have played it.
Warcraft 3: like with half-life 2 above, not many talk about 1/2.
Lol I keep adding to the list, wonder how big it will grow...