Killzone is an insanely good FPS for the PS2 and no player should be without.
Killzone takes place in the future with the Helghasts attacking the ISA forces. You have to defend the ISA and the planet with your squad. At first you take control of one man. During the missions you encounter new playable characters such as Luger and Rico. Each has their own unique abilities and also weaknesses. Rico has a powerful chaingun but walks a lot slower than the other characters. Luger has a knife and an automatic sniper pistol but has weak armor. Using the right character for the right scenario can play very well.
The multiplayer aspects in Killzone are quite fun as well. The online modes are very fun and with a fast connection play very very well. The offline battlegrounds mode also plays well if you and your friend want to play split screen against each other or with teams of bots, or just a 1P game with 14 bots against you. One of the downsides of Killzone's multiplayer is the characters movement speed and the extremely limited character selection (ie. either Helghast troop or ISA troop). The playing speed is very slow at sometimes when trying to cross a large canyon etc. and you can only use the sprint button when you have its energy full and it limits very quickly. Even then, it is not as fast as you would want it for a multi-player game and you cannot shoot, bash or throw grenades while sprinting.
Killzone can be somewhat hard at sometimes but with a few hours into the gameplay, it cools off and is a very exhiliarating FPS with only a few flaws.
The action is downright solid, reloading animations are very realistic, weapons are very accurately proportioned and designed, and the knife battles are extremely satisfying.
The graphics are insanely realistic and may have the best graphics of any FPS on the PS2. The voice acting is pretty well designed and the story is also pretty in-depth as well.
KillZone for the PS2 is possibly the most enjoyable and realistic FPS on the PS2 console to-date.