Call of Duty 4

[MetalMilitia]

It's actually a really solid game in both SP and MP. Sure the single player is short but it's extremely dense and not really that much shorter than some other new releases.

Mutiplayer is AMAZING in all caps. The rankings and achievements are great as you can unlock a whole bunch of content to use in custom MP classes. My favorite perk so far is definitely 'last stand' were instead of dieing when your health reaches zero you kind of fall over and pull out your sidearm giving you a ~20 second chance to shoot any nearby enemies (assuming the guy who originally killed you doesn't finish you off immediately).
Regular mode is nice and 'arcadey' while hardcore is, as the name would suggest, pretty realistic. Both are fun though.

I'd give it a 9/10. One mark being deducted for the really crappy weapon sounds.