Subject sounds too broad but humor me. I've been out of the gaming loop for awhile but looking forward to getting back into it. The last game series I really got into was Half Life 2 followed by about six months of WoW before I set games aside.
I'm building a new system in anticipation of Skyrim and SWTOR which aren't released for another few months. In the meantime I'd like to play something that will make the most of my new system. The COD series has basically passed me by and I've only played a little of World at War (mostly Nazi Zombies as I didn't like the campaign that much).
Play the CoD's, Crysis 2 which will tax your system pretty good, Battlefield Bad Company 2, Deus Ex: Human Revolution... there's a longer list, but you didn't say which kind of game you prefer (I assumed it's mostly FPS-type games from what you mentioned).
I'd recommend all the games you've listed.
COD4 MW is a great game with great storyline and gameplay.
COD6 MW2 is also requally good, but with a short single player campaign.
Crysis with it's outstanding graphics and gameplay.
Crysis 2 with good visuals and gameplay.
Metro 2033 with a dark atmosphere and a great storyline.
Basically, they're lots more.