The reason it destroys the competition. Thanfully Halo 4 is the same. 

I would have to murder someone if they changed the gameplay to COD style crap. 

I have played mostly good fps games, so the worst one is probably Goldeneye64 or maybe Half-life.

the last doom

Farcry 2 on the PS3.

I agree with you on Killzone 3. Terrible game, they went completely in the wrong direction with it. Also Far Cry 2 was just plain boring.

In terms of the COD games though, I think WaW had one of the worst campaign modes of any FPS game I've played ( I platinumed it, which was torture) for me Nazi Zombies was the only real highlight of a very poor game. I think the campaign modes of MW3 and BLOPS2 are far, far better.

I actually think the fact that it deviated from Halo 2 quite a bit made it a worse game in my eyes. Halo 2 is my favourite Halo game (Yet to play Halo 4 however.)

If I exclude demos, I'd say Modern Warfare 2 and 3 are tied.

I played Halo ODST for like an hour at a friends house and it was not my cup of tea (I played the older Halo games but I really don't have any recollection of them, nothing memorable). Mostly played single player and then like one match of multiplayer. I hate how some weapons can't aim down the sites...I just don't see how you can shoot accurately from a range without aiming down the sites. I also have some weird thing where I hate games that don't have actual guns in them (ex. Ak-47, M16 etc) and guns that don't shoot bullets but some random laser things or a made up projectile. Just a personal preference. Also I can't believe no one has mentioned Brink...That game was horrendous.

Army of two, 40th day (might have the name wrong)
The first one was pretty fun but this one was just lame from the get go.