I much prefer Insurgency: Modern Infantry Combat. Its a mod for Source, and it captures combat alot better than any of the CoD games, it's MP only though.
I played both CoD4 and World at War, neither of them where to my taste.
I liked the Hardcore game mode in CoD4 though, it was fairly realistic, one bullet kills you and such. But Insurgency does that alot better. And since Insurgency is only on PC you don't get alot of the Xbox kiddies with high gamer scores yelling at you when you kill them with a pistol.
This is the best way to describe how awsome Insurgency can be.
I played Insurgency with a group of actual soldiers a fue times, the tactics involved when these guys get into it is amazing. We where pinned down beside a gas station in baghdad, My commander told me to keep my head down and then he leaned out to check around the corner. He stepped out and we followed right accross the front windows of the store, with the gas pumps infront of us.
Suddenly a bullet whized past my head and took out the window, my commander got shot in the head and shoulder, and I went prone. I poped my head up in a crouched possition to see what was going on and an insurgent with an RPG7 took aim, I ducked and it went right over my head and into the window. The explosion shook everything and sent some random garbage items flying everywhere.
A fire fight broke out that lasted at least a fue minuts, I burned through six mags of ammunition just trying to keep the enemy's heads down. Since it doesn't tell you when you kill someone I don't actualy know how many I killed. I was stuck behind the front of a car untill reinforcements came and we where able to push them back.
That was one of the best experiances I have ever had in gaming.
But everyone can have own opinies 
