Sp or mp?
Sp mw2 is good but far too expensive for 5 hours game play so wait for it to be cheap
Cod4 is definitely excellent for SP and pretty good in MP as well, get it over 5 tbh.
L4d2 is excellent for MP, bit of a mix between conventional FPs and something new with the specials and there's an excellent ocuk community playing this with link providing servers. Hippos and Kearney are also currently making the l4d1 maps work in l4d2.
You can also play it purely FPs style in coop mode rather than vs
Defiantly my current favourite game.
The STALKER games (sp) are excellent too, my favourite is CS, but many prefer SOC. The lighting engine on these is second to none and with a few texture mods looks amazing and will really put your system to the test in terms of shadows and lighting.
Although the weapons you get at the start are very inaccurate (like any untrained guy will be with crappy soviet era cast offs

) so it;s not so much run and gun but get close enough that you can stick your sawn off shotgun up their arse ;p
Unconventional and excellent game play.
CS:S is still excellent for MP purely for game play rather than pretty gfx, but difficult to get into with most people being good shots by now.
Crysis warhead is excellent for visuals especially with some config mods. Many people really like the gameplay but for me it felt a bit hollow so maybe best to try a demo if you can find one.
And not an FPs but try Company of heroes and it's expansions for some visual lovleyness.
Hope this helps
