My favourite 18 rated game was on the playstation 2. It was called Cold...um...cold something. It was by Swordfish studios (odd, I remember the developer but not the name of the game. Its sitting not 3 feet from me in a drawer, but I'm too lazy to get it out.
It was basically like an 18 rated Goldeneye without the James Bond license, so they substituted ex-SAS/SIS dude. I thought it was absolutely awesome. Story was good, although the last level was a bit 'cloud base/hollowed out volcano, there was a couple of levels a bit like the escape from the KGB HQ in goldeneye, plus the obligatory middle east levels, a snow level like goldeneye too. Acting was good for a videogame. I rate it. If I could get it on the PC I would.
EDIT: Got it! Cold Winter! Seriously, I prefer it to CoD4, I wouldn't say it was a better game as such, but it's more my style. Its like Goldeneye. But with head explodey! And arm and leg rippy offy! And proper swearing delivered with and english accent. And some nice plot twists. Well, not so much a plot twist as, ooh, nasty, didnt see that coming...
PS Apologies for the poor grammer, post written by stream of thought.
EDIT 2: Actually, no, fuck it, it is a better game, I'm going to hook up my old Playstation 2 now and have some molotov lobbing, lockpicking, sneaky sneaky head explodey fun.