Charlie Don't Surf was the introduction a lot of people got into being in a modern military. The series' name, "Call of Duty" brings to mind (to me at least) that all men have an obligation to join the ranks and fight in the trenches for their country, which is what we saw with all of the WWII games. Charlie Don't Surf is the modern version of what that call looks like (albeit a somewhat Hollywood-ized version).
Modern Warfare 2
If you haven't seen Generation Kill you should. I'm pretty sure Infinity Ward used parts of that mini-series as inspiration for Team Player (as well as SSDD). Great pacing, excellent movement from action to quiet and back to frantic fight-for-your-life action.
This is the level where we are introduced to Soap. We played as him in CoD4, but here with Kevin McKidd's voice he instantly becomes a larger-than-life hero a la Captain Price (for me at least). And on top of that we get ice climbing, chasm jumping X2, stealthy snowstorm sneaking and spray-and-pray on a snowmobile. Two men take down an entire Russian air base.
A lot of CoD campaign missions have a quiet section, either a lull in the fighting or a stealth element to pace things better. This one though does it completely differently. It forces you to walk while your crew guns down civilians with some weird pulsing background music that creates a lot of tension while still being slow. It's a unique section with an atmosphere that I haven't seen in a video game before (not that I've played that many games, but I have played a few shooters).
I spent two years in Minas Gerais and went into a couple of small favelas while I was there so this level brought back some memories. From my experience they really captured the atmosphere, but beyond that the first time going through the level it seems like a massive maze of bricks and concrete while it's actually quite linear. I was very impressed that they could do that and guide the player through it in a way that doesn't disrupt the illusion. Plus it's an exciting level not knowing where you are going and having lots of enemies all around you all the time.
I want to take The Only Easy Day and The Gulag together because they take place one right after the other. I'm not sure I can really describe why I like them so much, but I think it's related to what I said for Charlie Don't Surf, except applied to a special forces team like the Navy SEALs. Of course that's on top of the fantastic gameplay.
Modern Warfare 3
Even though I didn't think Soap would die after MW2, the prologue of MW3 where Price is rushing him up to a doctor to get revived was pretty intense, so it was a satisfying relief to finally see that he was back in action here. After shadowing Price in CoD4 and Soap in MW2 here you are following along with both of them which makes you feel unstoppable.
MW3's story is much more straightforward compared to CoD4 and even MW2. Soap and Price are relentless in their pursuit of Makarov, so finally getting to the finish line, finally chasing him to the very end of the road is immensely gratifying. Yet it's also bitter-sweet. And the soundtrack really hits it home there.
Special Mention
The coup isn't really a campaign level in that you don't actually play, you just watch. But it's one of the best sequences in any game ever. This is one of the main reasons I hope we eventually get a CoD4 remake, to see The Coup again but with more detail, better lighting, better character animations, better facial expressions... I could go on but I won't. It was incredible for 2007, but imagine if it were redone for next-gen consoles in a few years...
