The Halo games were good, but they got repetitive after a while.
I liked Half-Life (and more so Half-Life 2) in terms of gameplay because it was always something new. You felt like you were moving forward all the time rather than needing to check the map cause you've stumbled over the same damn building again.
Personally I found Unreal 2 has one of the best stories. If only it wasn't as short as it was, and the characters weren't presented in a believable way. More in a way where you'd just know what was going on.
I've yet to find a game especially FPS that sucks me in totally.
This said Perfect Dark is by far the best FPS game I've ever played.
Single Player story was thin enough that you didn't feel like you had to remember everything, but there was enough of it to feel part of something.
The levels were very well designed never had you feel like it was the same old thing again and again.
Multiplayer was inherited from Goldeneye, although I think it would've been good to have the facility multiplayer map (BEST MAP EVER!) it was extremely strong with 4 players and 8 bots at once.
The bots which were fairly intelligent.
You also have all the little things that made the game great.
Graphically, it has light bloom, lens flare, field of depth, anti-aliasing, reflections, lightmapping, stencil shadows, high density particle effects.
Also the game had Primary, and Alternative Fire on all weapons. A quick selection menu, You could disarm people by shooting thier hands or even run up to them and take thier weapon. All levels you had the advantage of light, where you could shoot out all of the lights and this would allow you to walk around without enemies noticing you. The enemies weapons could jam, they would also have to reload.
On the whole the game was just truely a masterpeice
Halo and Half-Life ain't got nothing on Perfect Dark.