This is probably one of the first console shooters that actually played like a proper console shooter.Killzone
I think this game was probably aimed a little too big and even packed some gameplay elements that disappeared from the series after this title. In the original Killzone you could actually play as different characters that each had a different way of completing the mission. Guns blazing or being stealthy. The newer titles seem to try to combine all those elements into one character which actually takes away some of the replay value. But Killzone on PS2 pushed the console so hard that the game actually lagged at times. But it’s something that’s stuck with the series ever since: pushing the Playstation to its limits with graphics. Even today, Killzone 2 and 3 are two of the most amazing looking games on the PS3, much like Killzone was back on PS2. The game was also longer than most shooters today going about 10 hours depending on how difficult you found the game.
Like SOCOM, it was probably the most played online game on PS2. And it was damn fun for a console shooter with almost no network lag and even allowed for up to 16 players online. Something even some games today don’t offer… looking at your CoD. While it is largely regarded as overrated, and it is, it’s also a testament to what the PS2 was capable of.