1mm3 said:
Is that in any particular order?
Considering buying half life 2, loved the original
Vaguely although it's not necessarily in the order of greatness

Here's a bit more info:
Far Cry: Has nearly everything - great enemies, great weapons, huge varied levels, relatively open/non-linear (for a FPS), fantastic graphics despite being over 3 years old, and is quite a long game.
Doom3+expansion: Good atmosphere and surprisingly good plot development. Again, a fairly long game.
Halflife2+ep1: Strong characterisation and immersion, a good bet as you loved HL.
CoD Deluxe: Incredibly atmospheric, great use of sound and had me hooked from start to finish. The only FPS I've ever completed in less than a day after installing it. Extremely polished game.
CoD2: Similar to the above, better graphics and slightly worse atmosphere.
MOHPA: Nice sense of storyline and characterisation through the use of AI teammate orders and cutscenes. Again, great atmosphere (right down to the main menu) although let down slightly by some lacklustre jungle levels. Good variety though.
Unreal: Absolutely huge game with very good AI and some fantastic set pieces especially in the early levels (I'll never forget my first encounter with a Skaarj). May seem slightly dated now especially as it has no clear objectives/compass/icons to guide you.
Deus Ex: Listed last because it's not really a FPS in the traditional sense, not due to lack of quality. Absolutely stunning FPS-RPG hybrid which is yet to be bettered some 7 years on. Very open ended and suits a variety of playstyles.
...and a couple I forgot:
Serious Sam Gold / 2: Crazy fun blast fests with a heavy dose of humour. If you are sick of modern games only serving up half a dozen enemies at once, give these a bash.
Painkiller+expansion: Similar to the above, although much darker in style with the humour removed. It's actually quite creepy in places. One of the best selections of weapons in any game.